Method and device for encoding/decoding image, and recording medium in which bitstream is stored

ABSTRACT

An image encoding/decoding method and apparatus for performing intra prediction using a plurality of reference sample lines are provided. An image decoding method may comprise configuring a plurality of reference sample lines, reconstructing an intra prediction mode of a current block, and performing intra prediction for the current block based on the intra prediction mode and the plurality of reference sample lines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/628,850, filed Jan. 6, 2020, now allowed, which is a U.S. NationalStage Application of International Application No. PCT/KR2018/007016,filed on Jun. 21, 2018, which claims the benefit under 35 USC 119(a) and365(b) of Korean Patent Application No. 10-2017-0086113, filed on Jul.6, 2017, Korean Patent Application No. 10-2017-0171377, filed on Dec.13, 2017 in the Korean Intellectual Property Office, the disclosures ofwhich are all incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method and apparatus forencoding/decoding an image and a recording medium storing a bitstream.Particularly, the present invention relates to a method and apparatusfor encoding/decoding an image using intra prediction and a recordingmedium storing a bitstream generated by an image encodingmethod/apparatus of the present invention.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images, haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques arerequired for higher-resolution and higher-quality images.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; a transform and quantization technique for compressing energyof a residual signal; an entropy encoding technique of assigning a shortcode to a value with a high appearance frequency and assigning a longcode to a value with a low appearance frequency; etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and apparatusfor encoding and decoding an image to enhance compression efficiency.

Another object of the present invention is to provide a method andapparatus for encoding and decoding an image using intra prediction toenhance compression efficiency.

Another object of the present invention is to provide a recording mediumstoring a bitstream generated by an image encoding method/apparatus ofthe present invention.

Technical Solution

According to the present invention, an image decoding method comprisingconfiguring a plurality of reference sample lines, reconstructing anintra prediction mode of a current block, and performing intraprediction for the current block based on the intra prediction mode andthe plurality of reference sample lines may be provided.

In the image decoding method according to the present invention, theconfiguring the plurality of reference sample lines may perform paddingon a reference sample located at a predetermined position withoutdetermining availability of the reference sample.

In the image decoding method according to the present invention, when alength of a horizontal side and a length of a vertical side of thecurrent block are W and H, respectively, an x coordinate or a ycoordinate of the predetermined position may be equal to greater thanW+H.

In the image decoding method according to the present invention, thepadding may be performed using a reference sample adjacent to thepredetermined position, the reference sample being located at a positionhaving an x coordinate or a y coordinate being W+H−1.

In the image decoding method according to the present invention, theconfiguring the plurality of reference sample lines may compriseperforming filtering on each of the plurality of reference sample lines.

In the image decoding method according to the present invention, atleast one of whether to apply the filtering and a filter type may beadaptively determined based on at least one of an intra prediction mode,a size, a shape of the current block and a target reference sample line.

In the image decoding method according to the present invention, anumber of the plurality of reference sample lines may be adaptivelydetermined based on an intra prediction mode of the current block.

In the image decoding method according to the present invention, anumber of the plurality of reference sample lines may be adaptivelydetermined based on whether a left boundary or a top boundary of thecurrent block corresponds to a boundary of a predetermined image region.

In the image decoding method according to the present invention, asingle reference sample line may be used for samples in a left side ofthe current block when the left boundary of the current block is theboundary of the predetermined image region, and a single referencesample line may be used for samples in an upper side of the currentblock when the top boundary of the current block is the boundary of thepredetermined image region.

In the image decoding method according to the present invention, thepredetermined image region may be at least one of a picture, a tile, aslice, and a coding tree block (CTB).

Further, according to the present invention, an image encoding methodcomprising determining an intra prediction mode of a current block,configuring a plurality of reference sample lines, and performing intraprediction for the current block based on the intra prediction mode andthe plurality of reference sample lines may be provided.

In the image encoding method according to the present invention, theconfiguring the plurality of reference sample lines may perform paddingon a reference sample located at a predetermined position withoutdetermining availability of the reference sample.

In the image encoding method according to the present invention, when alength of a horizontal side and a length of a vertical side of thecurrent block are W and H, respectively, an x coordinate or a ycoordinate of the predetermined position may be equal to greater thanW+H.

In the image encoding method according to the present invention, thepadding may be performed using a reference sample adjacent to thepredetermined position, the reference sample being located at a positionhaving an x coordinate or a y coordinate being W+H−1.

In the image encoding method according to the present invention, theconfiguring the plurality of reference sample lines may compriseperforming filtering on each of the plurality of reference sample lines.

In the image encoding method according to the present invention, atleast one of whether to apply the filtering and a filter type may beadaptively determined based on at least one of an intra prediction mode,a size, a shape of the current block and a target reference sample line.

In the image encoding method according to the present invention, anumber of the plurality of reference sample lines may be adaptivelydetermined based on an intra prediction mode of the current block.

In the image encoding method according to the present invention, anumber of the plurality of reference sample lines may be adaptivelydetermined based on whether a left boundary or a top boundary of thecurrent block corresponds to a boundary of a predetermined image region.

In the image encoding method according to the present invention, asingle reference sample line may be used for samples in a left side ofthe current block when the left boundary of the current block is theboundary of the predetermined image region, and a single referencesample line may be used for samples in an upper side of the currentblock when the top boundary of the current block is the boundary of thepredetermined image region.

Further, a recording medium according to the present invention may storea bitstream generated by an image encoding method according to thepresent invention.

Advantageous Effects

According to the present invention, a method and apparatus for encodingand decoding an image to enhance compression efficiency may be provided.

According to the present invention, a method and apparatus for encodingand decoding an image using intra prediction to enhance compressionefficiency may be provided.

According to the present invention, a recording medium storing abitstream generated by an image encoding method/apparatus of the presentinvention may be provided.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing configurations of an encodingapparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing configurations of a decoding apparatusaccording to an embodiment of the present invention.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image.

FIG. 4 is a view for explaining an embodiment of a process of intraprediction.

FIG. 5 is a view for explaining intra prediction according to thepresent invention.

FIG. 6 is an exemplary diagram illustrating the relationship between aluma block and a chroma block.

FIG. 7 is a diagram for describing a plurality of reconstructed samplelines.

FIG. 8 is a diagram for describing a process of replacing an unavailablesample with an available sample.

FIG. 9 illustrates various filter shapes.

FIG. 10 is a diagram for describing intra prediction according to theshapes of the current block.

FIG. 11 is a diagram illustrating an embodiment in which two referencesample lines are used.

FIG. 12 is a diagram for describing neighboring samples of a currentblock used to derive a parameter of linear models which are used forpredicting a chroma component from a luma component.

FIG. 13 is an exemplary diagram illustrating a process of restructuringa color component block.

FIG. 14 is a diagram illustrating an embodiment performing restructuringby using a plurality of upper-side reference sample lines and/or aplurality of left-side reference sample lines.

FIG. 15 is an exemplary diagram illustrating reference samples used forthe restructuring in accordance with an intra prediction mode or acoding parameter of a corresponding block.

FIG. 16 is a diagram illustrating an exemplary restructured first colorcomponent corresponding block when a second color component predictiontarget block is a 4×4 block.

FIG. 17 is a diagram illustrating a sample of a first color componentand a sample of a second color component.

MODE FOR CARRYING OUT THE INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, althoughthe exemplary embodiments can be construed as including allmodifications, equivalents, or substitutes in a technical concept and atechnical scope of the present invention. The similar reference numeralsrefer to the same or similar functions in various aspects. In thedrawings, the shapes and dimensions of elements may be exaggerated forclarity. In the following detailed description of the present invention,references are made to the accompanying drawings that show, by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to implement the present disclosure. Itshould be understood that various embodiments of the present disclosure,although different, are not necessarily mutually exclusive. For example,specific features, structures, and characteristics described herein, inconnection with one embodiment, may be implemented within otherembodiments without departing from the spirit and scope of the presentdisclosure. In addition, it should be understood that the location orarrangement of individual elements within each disclosed embodiment maybe modified without departing from the spirit and scope of the presentdisclosure. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present disclosure isdefined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to what the claims claim.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

Furthermore, constitutional parts shown in the embodiments of thepresent invention are independently shown so as to representcharacteristic functions different from each other. Thus, it does notmean that each constitutional part is constituted in a constitutionalunit of separated hardware or software. In other words, eachconstitutional part includes each of enumerated constitutional parts forconvenience. Thus, at least two constitutional parts of eachconstitutional part may be combined to form one constitutional part orone constitutional part may be divided into a plurality ofconstitutional parts to perform each function. The embodiment where eachconstitutional part is combined and the embodiment where oneconstitutional part is divided are also included in the scope of thepresent invention, if not departing from the essence of the presentinvention.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded. In other words, when a specific element is referred to as being“included”, elements other than the corresponding element are notexcluded, but additional elements may be included in embodiments of thepresent invention or the scope of the present invention.

In addition, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In describingexemplary embodiments of the present invention, well-known functions orconstructions will not be described in detail since they mayunnecessarily obscure the understanding of the present invention. Thesame constituent elements in the drawings are denoted by the samereference numerals, and a repeated description of the same elements willbe omitted.

Hereinafter, an image may mean a picture configuring a video, or maymean the video itself. For example, “encoding or decoding or both of animage” may mean “encoding or decoding or both of a moving picture”, andmay mean “encoding or decoding or both of one image among images of amoving picture.”

Hereinafter, terms “moving picture” and “video” may be used as the samemeaning and be replaced with each other.

Hereinafter, a target image may be an encoding target image which is atarget of encoding and/or a decoding target image which is a target ofdecoding. Also, a target image may be an input image inputted to anencoding apparatus, and an input image inputted to a decoding apparatus.Here, a target image may have the same meaning with the current image.

Hereinafter, terms “image”, “picture, “frame” and “screen” may be usedas the same meaning and be replaced with each other.

Hereinafter, a target block may be an encoding target block which is atarget of encoding and/or a decoding target block which is a target ofdecoding. Also, a target block may be the current block which is atarget of current encoding and/or decoding. For example, terms “targetblock” and “current block” may be used as the same meaning and bereplaced with each other.

Hereinafter, terms “block” and “unit” may be used as the same meaningand be replaced with each other. Or a “block” may represent a specificunit.

Hereinafter, terms “region” and “segment” may be replaced with eachother.

Hereinafter, a specific signal may be a signal representing a specificblock. For example, an original signal may be a signal representing atarget block. A prediction signal may be a signal representing aprediction block. A residual signal may be a signal representing aresidual block.

In embodiments, each of specific information, data, flag, index, elementand attribute, etc. may have a value. A value of information, data,flag, index, element and attribute equal to “0” may represent a logicalfalse or the first predefined value. In other words, a value “0”, afalse, a logical false and the first predefined value may be replacedwith each other. A value of information, data, flag, index, element andattribute equal to “1” may represent a logical true or the secondpredefined value. In other words, a value “1”, a true, a logical trueand the second predefined value may be replaced with each other.

When a variable i or j is used for representing a column, a row or anindex, a value of i may be an integer equal to or greater than 0, orequal to or greater than 1. That is, the column, the row, the index,etc. may be counted from 0 or may be counted from 1.

Description of Terms

Encoder: means an apparatus performing encoding. That is, means anencoding apparatus.

Decoder: means an apparatus performing decoding. That is, means andecoding apparatus.

Block: is an M×N array of a sample. Herein, M and N may mean positiveintegers, and the block may mean a sample array of a two-dimensionalform. The block may refer to a unit. A current block my mean an encodingtarget block that becomes a target when encoding, or a decoding targetblock that becomes a target when decoding. In addition, the currentblock may be at least one of an encode block, a prediction block, aresidual block, and a transform block.

Sample: is a basic unit constituting a block. It may be expressed as avalue from 0 to 2^(Bd)−1 according to a bit depth (Bd). In the presentinvention, the sample may be used as a meaning of a pixel. That is, asample, a pel, a pixel may have the same meaning with each other.

Unit: may refer to an encoding and decoding unit. When encoding anddecoding an image, the unit may be a region generated by partitioning asingle image. In addition, the unit may mean a subdivided unit when asingle image is partitioned into subdivided units during encoding ordecoding. That is, an image may be partitioned into a plurality ofunits. When encoding and decoding an image, a predetermined process foreach unit may be performed. A single unit may be partitioned intosub-units that have sizes smaller than the size of the unit. Dependingon functions, the unit may mean a block, a macroblock, a coding treeunit, a code tree block, a coding unit, a coding block), a predictionunit, a prediction block, a residual unit), a residual block, atransform unit, a transform block, etc. In addition, in order todistinguish a unit from a block, the unit may include a luma componentblock, a chroma component block associated with the luma componentblock, and a syntax element of each color component block. The unit mayhave various sizes and forms, and particularly, the form of the unit maybe a two-dimensional geometrical figure such as a square shape, arectangular shape, a trapezoid shape, a triangular shape, a pentagonalshape, etc. In addition, unit information may include at least one of aunit type indicating the coding unit, the prediction unit, the transformunit, etc., and a unit size, a unit depth, a sequence of encoding anddecoding of a unit, etc.

Coding Tree Unit: is configured with a single coding tree block of aluma component Y, and two coding tree blocks related to chromacomponents Cb and Cr. In addition, it may mean that including the blocksand a syntax element of each block. Each coding tree unit may bepartitioned by using at least one of a quad-tree partitioning method, abinary-tree partitioning method, a ternary-tree partitioning method,etc. to configure a lower unit such as coding unit, prediction unit,transform unit, etc. It may be used as a term for designating a sampleblock that becomes a process unit when encoding/decoding an image as aninput image. Here, a quad-tree may mean a quarternary-tree.

When the size of a coding block falls within a first predeterminedrange, only quad-tree partitioning is allowed for the coding block.Here, the first predetermined range may be defined by at least one of amaximum size and a minimum size of a coding block that can bepartitioned only by quad-tree partitioning. Information indicating themaximum/minimum size of the coding block for which quad-treepartitioning is allowed may be signaled as data included in a bitstream,and the information may be signaled in units of at least one of asequence, a picture parameter, and a slice (segment). Alternatively, themaximum/minimum size of the coding block may be a fixed size preset inthe encoder/decoder. For example, when the size of the coding block iswithin a range from 64×64 to 256×256, the coding block can bepartitioned only by quad-tree partitioning. Alternatively, when the sizeof the coding block is larger than the maximum size of a transform block(TB), the coding block can be partitioned only by quad-treepartitioning. In this case, the block to be partitioned into quadrantsmay be either a coding block or a transform block. In this case,information (for example, split flag) indicating the quad-treepartitioning of a coding block may be a flag indicating whether or notthe coding unit is partitioned by quad-tree partitioning. When the sizeof a coding block falls within a second predetermined range, the codingblock can be partitioned only by binary-tree partitioning orternary-tree partitioning. In this case, the above description ofquad-tree partitioning can also be applied to binary-tree partitioningor ternary-tree partitioning.

Coding Tree Block: may be used as a term for designating any one of a Ycoding tree block, Cb coding tree block, and Cr coding tree block.

Neighbor Block: may mean a block adjacent to a current block. The blockadjacent to the current block may mean a block that comes into contactwith a boundary of the current block, or a block positioned within apredetermined distance from the current block. The neighbor block maymean a block adjacent to a vertex of the current block. Herein, theblock adjacent to the vertex of the current block may mean a blockvertically adjacent to a neighbor block that is horizontally adjacent tothe current block, or a block horizontally adjacent to a neighbor blockthat is vertically adjacent to the current block.

Reconstructed Neighbor block: may mean a neighbor block adjacent to acurrent block and which has been already spatially/temporally encoded ordecoded. Herein, the reconstructed neighbor block may mean areconstructed neighbor unit. A reconstructed spatial neighbor block maybe a block within a current picture and which has been alreadyreconstructed through encoding or decoding or both. A reconstructedtemporal neighbor block is a block at a corresponding position as thecurrent block of the current picture within a reference image, or aneighbor block thereof.

Unit Depth: may mean a partitioned degree of a unit. In a treestructure, the highest node (Root Node) may correspond to the first unitwhich is not partitioned. Also, the highest node may have the leastdepth value. In this case, the highest node may have a depth of level 0.A node having a depth of level 1 may represent a unit generated bypartitioning once the first unit. A node having a depth of level 2 mayrepresent a unit generated by partitioning twice the first unit. A nodehaving a depth of level n may represent a unit generated by partitioningn-times the first unit. A Leaf Node may be the lowest node and a nodewhich cannot be partitioned further. A depth of a Leaf Node may be themaximum level. For example, a predefined value of the maximum level maybe 3. A depth of a root node may be the lowest and a depth of a leafnode may be the deepest. In addition, when a unit is expressed as a treestructure, a level in which a unit is present may mean a unit depth.

Bitstream: may mean a bitstream including encoding image information.

Parameter Set: corresponds to header information among a configurationwithin a bitstream. At least one of a video parameter set, a sequenceparameter set, a picture parameter set, and an adaptation parameter setmay be included in a parameter set. In addition, a parameter set mayinclude a slice header, and tile header information.

Parsing: may mean determination of a value of a syntax element byperforming entropy decoding, or may mean the entropy decoding itself.

Symbol: may mean at least one of a syntax element, a coding parameter,and a transform coefficient value of an encoding/decoding target unit.In addition, the symbol may mean an entropy encoding target or anentropy decoding result.

Prediction Mode: may be information indicating a mode encoded/decodedwith intra prediction or a mode encoded/decoded with inter prediction.

Prediction Unit: may mean a basic unit when performing prediction suchas inter-prediction, intra-prediction, inter-compensation,intra-compensation, and motion compensation. A single prediction unitmay be partitioned into a plurality of partitions having a smaller size,or may be partitioned into a plurality of lower prediction units. Aplurality of partitions may be a basic unit in performing prediction orcompensation. A partition which is generated by dividing a predictionunit may also be a prediction unit.

Prediction Unit Partition: may mean a form obtained by partitioning aprediction unit.

Transform Unit: may mean a basic unit when performing encoding/decodingsuch as transform, inverse-transform, quantization, dequantization,transform coefficient encoding/decoding of a residual signal. A singletransform unit may be partitioned into a plurality of lower-leveltransform units having a smaller size. Here,transformation/inverse-transformation may comprise at least one amongthe first transformation/the first inverse-transformation and the secondtransformation/the second inverse-transformation.

Scaling: may mean a process of multiplying a quantized level by afactor. A transform coefficient may be generated by scaling a quantizedlevel. The scaling also may be referred to as dequantization.

Quantization Parameter: may mean a value used when generating aquantized level using a transform coefficient during quantization. Thequantization parameter also may mean a value used when generating atransform coefficient by scaling a quantized level duringdequantization. The quantization parameter may be a value mapped on aquantization step size.

Delta Quantization Parameter: may mean a difference value between apredicted quantization parameter and a quantization parameter of anencoding/decoding target unit.

Scan: may mean a method of sequencing coefficients within a unit, ablock or a matrix. For example, changing a two-dimensional matrix ofcoefficients into a one-dimensional matrix may be referred to asscanning, and changing a one-dimensional matrix of coefficients into atwo-dimensional matrix may be referred to as scanning or inversescanning.

Transform Coefficient: may mean a coefficient value generated aftertransform is performed in an encoder. It may mean a coefficient valuegenerated after at least one of entropy decoding and dequantization isperformed in a decoder. A quantized level obtained by quantizing atransform coefficient or a residual signal, or a quantized transformcoefficient level also may fall within the meaning of the transformcoefficient.

Quantized Level: may mean a value generated by quantizing a transformcoefficient or a residual signal in an encoder. Alternatively, thequantized level may mean a value that is a dequantization target toundergo dequantization in a decoder. Similarly, a quantized transformcoefficient level that is a result of transform and quantization alsomay fall within the meaning of the quantized level.

Non-zero Transform Coefficient: may mean a transform coefficient havinga value other than zero, or a transform coefficient level or a quantizedlevel having a value other than zero.

Quantization Matrix: may mean a matrix used in a quantization process ora dequantization process performed to improve subjective or objectiveimage quality. The quantization matrix also may be referred to as ascaling list.

Quantization Matrix Coefficient: may mean each element within aquantization matrix. The quantization matrix coefficient also may bereferred to as a matrix coefficient.

Default Matrix: may mean a predetermined quantization matrixpreliminarily defined in an encoder or a decoder.

Non-default Matrix: may mean a quantization matrix that is notpreliminarily defined in an encoder or a decoder but is signaled by auser.

Statistic Value: a statistic value for at least one among a variable, anencoding parameter, a constant value, etc. which have a computablespecific value may be one or more among an average value, a weightedaverage value, a weighted sum value, the minimum value, the maximumvalue, the most frequent value, a median value, an interpolated value ofthe corresponding specific values.

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatus,or an image encoding apparatus. A video may include at least one image.The encoding apparatus 100 may sequentially encode at least one image.

Referring to FIG. 1 , the encoding apparatus 100 may include a motionprediction unit 111, a motion compensation unit 112, an intra-predictionunit 120, a switch 115, a subtractor 125, a transform unit 130, aquantization unit 140, an entropy encoding unit 150, a dequantizationunit 160, a inverse-transform unit 170, an adder 175, a filter unit 180,and a reference picture buffer 190.

The encoding apparatus 100 may perform encoding of an input image byusing an intra mode or an inter mode or both. In addition, encodingapparatus 100 may generate a bitstream including encoded informationthrough encoding the input image, and output the generated bitstream.The generated bitstream may be stored in a computer readable recordingmedium, or may be streamed through a wired/wireless transmission medium.When an intra mode is used as a prediction mode, the switch 115 may beswitched to an intra. Alternatively, when an inter mode is used as aprediction mode, the switch 115 may be switched to an inter mode.Herein, the intra mode may mean an intra-prediction mode, and the intermode may mean an inter-prediction mode. The encoding apparatus 100 maygenerate a prediction block for an input block of the input image. Inaddition, the encoding apparatus 100 may encode a residual block using aresidual of the input block and the prediction block after theprediction block being generated. The input image may be called as acurrent image that is a current encoding target. The input block may becalled as a current block that is current encoding target, or as anencoding target block.

When a prediction mode is an intra mode, the intra-prediction unit 120may use a sample of a block that has been already encoded/decoded and isadjacent to a current block as a reference sample. The intra-predictionunit 120 may perform spatial prediction for the current block by using areference sample, or generate prediction samples of an input block byperforming spatial prediction. Herein, the intra prediction may meanintra-prediction,

When a prediction mode is an inter mode, the motion prediction unit 111may retrieve a region that best matches with an input block from areference image when performing motion prediction, and deduce a motionvector by using the retrieved region. In this case, a search region maybe used as the region. The reference image may be stored in thereference picture buffer 190. Here, when encoding/decoding for thereference image is performed, it may be stored in the reference picturebuffer 190.

The motion compensation unit 112 may generate a prediction block byperforming motion compensation for the current block using a motionvector. Herein, inter-prediction may mean inter-prediction or motioncompensation.

When the value of the motion vector is not an integer, the motionprediction unit 111 and the motion compensation unit 112 may generatethe prediction block by applying an interpolation filter to a partialregion of the reference picture. In order to perform inter prediction ormotion compensation on a coding unit, it may be determined that whichmode among a skip mode, a merge mode, an advanced motion vectorprediction (AMVP) mode, and a current picture referring mode is used formotion prediction and motion compensation of a prediction unit includedin the corresponding coding unit. Then, inter prediction or motioncompensation may be differently performed depending on the determinedmode.

The subtractor 125 may generate a residual block by using a residual ofan input block and a prediction block. The residual block may be calledas a residual signal. The residual signal may mean a difference betweenan original signal and a prediction signal. In addition, the residualsignal may be a signal generated by transforming or quantizing, ortransforming and quantizing a difference between the original signal andthe prediction signal. The residual block may be a residual signal of ablock unit.

The transform unit 130 may generate a transform coefficient byperforming transform of a residual block, and output the generatedtransform coefficient. Herein, the transform coefficient may be acoefficient value generated by performing transform of the residualblock. When a transform skip mode is applied, the transform unit 130 mayskip transform of the residual block.

A quantized level may be generated by applying quantization to thetransform coefficient or to the residual signal. Hereinafter, thequantized level may be also called as a transform coefficient inembodiments.

The quantization unit 140 may generate a quantized level by quantizingthe transform coefficient or the residual signal according to aparameter, and output the generated quantized level. Herein, thequantization unit 140 may quantize the transform coefficient by using aquantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingentropy encoding according to a probability distribution on valuescalculated by the quantization unit 140 or on coding parameter valuescalculated when performing encoding, and output the generated bitstream.The entropy encoding unit 150 may perform entropy encoding of sampleinformation of an image and information for decoding an image. Forexample, the information for decoding the image may include a syntaxelement.

When entropy encoding is applied, symbols are represented so that asmaller number of bits are assigned to a symbol having a high chance ofbeing generated and a larger number of bits are assigned to a symbolhaving a low chance of being generated, and thus, the size of bit streamfor symbols to be encoded may be decreased. The entropy encoding unit150 may use an encoding method for entropy encoding such as exponentialGolomb, context-adaptive variable length coding (CAVLC),context-adaptive binary arithmetic coding (CABAC), etc. For example, theentropy encoding unit 150 may perform entropy encoding by using avariable length coding/code (VLC) table. In addition, the entropyencoding unit 150 may deduce a binarization method of a target symboland a probability model of a target symbol/bin, and perform arithmeticcoding by using the deduced binarization method, and a context model.

In order to encode a transform coefficient level (quantized level), theentropy encoding unit 150 may change a two-dimensional block formcoefficient into a one-dimensional vector form by using a transformcoefficient scanning method.

A coding parameter may include information (flag, index, etc.) such assyntax element that is encoded in an encoder and signaled to a decoder,and information derived when performing encoding or decoding. The codingparameter may mean information required when encoding or decoding animage. For example, at least one value or a combination form of aunit/block size, a unit/block depth, unit/block partition information,unit/block shape, unit/block partition structure, whether to partitionof a quad-tree form, whether to partition of a binary-tree form, apartition direction of a binary-tree form (horizontal direction orvertical direction), a partition form of a binary-tree form (symmetricpartition or asymmetric partition), whether to partition of aternary-tree form, a partition direction of a ternary-tree form(horizontal direction or vertical direction), a partition form of aternary-tree form (symmetric partition or asymmetric partition), whetherto partition of a multi-type-tree form, a partition direction of amulti-type-tree form (horizontal direction or vertical direction), apartition form of a multi-type-tree form (symmetric partition orasymmetric partition), a partitioning tree of multi-type-tree form, aprediction mode (intra prediction or inter prediction), a lumaintra-prediction mode/direction, a chroma intra-predictionmode/direction, intra partition information, inter partitioninformation, a coding block partition flag, a prediction block partitionflag, a transform block partition flag, a reference sample filteringmethod, a reference sample filter tab, a reference sample filtercoefficient, a prediction block filtering method, a prediction blockfilter tap, a prediction block filter coefficient, a prediction blockboundary filtering method, a prediction block boundary filter tab, aprediction block boundary filter coefficient, an intra-prediction mode,an inter-prediction mode, motion information, a motion vector, a motionvector difference, a reference picture index, a inter-prediction angle,an inter-prediction indicator, a prediction list utilization flag, areference picture list, a reference picture, a motion vector predictorindex, a motion vector predictor candidate, a motion vector candidatelist, whether to use a merge mode, a merge index, a merge candidate, amerge candidate list, whether to use a skip mode, an interpolationfilter type, an interpolation filter tab, an interpolation filtercoefficient, a motion vector size, a presentation accuracy of a motionvector, a transform type, a transform size, information of whether ornot a primary (first) transform is used, information of whether or not asecondary transform is used, a primary transform index, a secondarytransform index, information of whether or not a residual signal ispresent, a coded block pattern, a coded block flag (CBF), a quantizationparameter, a quantization parameter residue, a quantization matrix,whether to apply an intra loop filter, an intra loop filter coefficient,an intra loop filter tab, an intra loop filter shape/form, whether toapply a deblocking filter, a deblocking filter coefficient, a deblockingfilter tab, a deblocking filter strength, a deblocking filtershape/form, whether to apply an adaptive sample offset, an adaptivesample offset value, an adaptive sample offset category, an adaptivesample offset type, whether to apply an adaptive loop filter, anadaptive loop filter coefficient, an adaptive loop filter tab, anadaptive loop filter shape/form, a binarization/inverse-binarizationmethod, a context model determining method, a context model updatingmethod, whether to perform a regular mode, whether to perform a bypassmode, a context bin, a bypass bin, a significant coefficient flag, alast significant coefficient flag, a coded flag for a unit of acoefficient group, a position of the last significant coefficient, aflag for whether a value of a coefficient is larger than 1, a flag forwhether a value of a coefficient is larger than 2, a flag for whether avalue of a coefficient is larger than 3, information on a remainingcoefficient value, a sign information, a reconstructed luma sample, areconstructed chroma sample, a residual luma sample, a residual chromasample, a luma transform coefficient, a chroma transform coefficient, aquantized luma level, a quantized chroma level, a transform coefficientlevel scanning method, a size of a motion vector search area at adecoder side, a shape of a motion vector search area at a decoder side,a number of time of a motion vector search at a decoder side,information on a CTU size, information on a minimum block size,information on a maximum block size, information on a maximum blockdepth, information on a minimum block depth, an imagedisplaying/outputting sequence, slice identification information, aslice type, slice partition information, tile identificationinformation, a tile type, tile partition information, a picture type, abit depth of an input sample, a bit depth of a reconstruction sample, abit depth of a residual sample, a bit depth of a transform coefficient,a bit depth of a quantized level, and information on a luma signal orinformation on a chroma signal may be included in the coding parameter.

Herein, signaling the flag or index may mean that a corresponding flagor index is entropy encoded and included in a bitstream by an encoder,and may mean that the corresponding flag or index is entropy decodedfrom a bitstream by a decoder.

When the encoding apparatus 100 performs encoding throughinter-prediction, an encoded current image may be used as a referenceimage for another image that is processed afterwards. Accordingly, theencoding apparatus 100 may reconstruct or decode the encoded currentimage, or store the reconstructed or decoded image as a reference imagein reference picture buffer 190.

A quantized level may be dequantized in the dequantization unit 160, ormay be inverse-transformed in the inverse-transform unit 170. Adequantized or inverse-transformed coefficient or both may be added witha prediction block by the adder 175. By adding the dequantized orinverse-transformed coefficient or both with the prediction block, areconstructed block may be generated. Herein, the dequantized orinverse-transformed coefficient or both may mean a coefficient on whichat least one of dequantization and inverse-transform is performed, andmay mean a reconstructed residual block.

A reconstructed block may pass through the filter unit 180. The filterunit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to areconstructed sample, a reconstructed block or a reconstructed image.The filter unit 180 may be called as an in-loop filter.

The deblocking filter may remove block distortion generated inboundaries between blocks. In order to determine whether or not to applya deblocking filter, whether or not to apply a deblocking filter to acurrent block may be determined based samples included in several rowsor columns which are included in the block. When a deblocking filter isapplied to a block, another filter may be applied according to arequired deblocking filtering strength.

In order to compensate an encoding error, a proper offset value may beadded to a sample value by using a sample adaptive offset. The sampleadaptive offset may correct an offset of a deblocked image from anoriginal image by a sample unit. A method of partitioning samples of animage into a predetermined number of regions, determining a region towhich an offset is applied, and applying the offset to the determinedregion, or a method of applying an offset in consideration of edgeinformation on each sample may be used.

The adaptive loop filter may perform filtering based on a comparisonresult of the filtered reconstructed image and the original image.Samples included in an image may be partitioned into predeterminedgroups, a filter to be applied to each group may be determined, anddifferential filtering may be performed for each group. Information ofwhether or not to apply the ALF may be signaled by coding units (CUs),and a form and coefficient of the ALF to be applied to each block mayvary.

The reconstructed block or the reconstructed image having passed throughthe filter unit 180 may be stored in the reference picture buffer 190. Areconstructed block processed by the filter unit 180 may be a part of areference image. That is, a reference image is a reconstructed imagecomposed of reconstructed blocks processed by the filter unit 180. Thestored reference image may be used later in inter prediction or motioncompensation.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

A decoding apparatus 200 may a decoder, a video decoding apparatus, oran image decoding apparatus.

Referring to FIG. 2 , the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization unit 220, a inverse-transform unit230, an intra-prediction unit 240, a motion compensation unit 250, anadder 225, a filter unit 260, and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer readable recording medium, or may receivea bitstream that is streamed through a wired/wireless transmissionmedium. The decoding apparatus 200 may decode the bitstream by using anintra mode or an inter mode. In addition, the decoding apparatus 200 maygenerate a reconstructed image generated through decoding or a decodedimage, and output the reconstructed image or decoded image.

When a prediction mode used when decoding is an intra mode, a switch maybe switched to an intra. Alternatively, when a prediction mode used whendecoding is an inter mode, a switch may be switched to an inter mode.

The decoding apparatus 200 may obtain a reconstructed residual block bydecoding the input bitstream, and generate a prediction block. When thereconstructed residual block and the prediction block are obtained, thedecoding apparatus 200 may generate a reconstructed block that becomes adecoding target by adding the reconstructed residual block with theprediction block. The decoding target block may be called a currentblock.

The entropy decoding unit 210 may generate symbols by entropy decodingthe bitstream according to a probability distribution. The generatedsymbols may include a symbol of a quantized level form. Herein, anentropy decoding method may be a inverse-process of the entropy encodingmethod described above.

In order to decode a transform coefficient level (quantized level), theentropy decoding unit 210 may change a one-directional vector formcoefficient into a two-dimensional block form by using a transformcoefficient scanning method.

A quantized level may be dequantized in the dequantization unit 220, orinverse-transformed in the inverse-transform unit 230. The quantizedlevel may be a result of dequantizing or inverse-transforming or both,and may be generated as a reconstructed residual block. Herein, thedequantization unit 220 may apply a quantization matrix to the quantizedlevel.

When an intra mode is used, the intra-prediction unit 240 may generate aprediction block by performing, for the current block, spatialprediction that uses a sample value of a block adjacent to a decodingtarget block and which has been already decoded.

When an inter mode is used, the motion compensation unit 250 maygenerate a prediction block by performing, for the current block, motioncompensation that uses a motion vector and a reference image stored inthe reference picture buffer 270.

The adder 225 may generate a reconstructed block by adding thereconstructed residual block with the prediction block. The filter unit260 may apply at least one of a deblocking filter, a sample adaptiveoffset, and an adaptive loop filter to the reconstructed block orreconstructed image. The filter unit 260 may output the reconstructedimage. The reconstructed block or reconstructed image may be stored inthe reference picture buffer 270 and used when performinginter-prediction. A reconstructed block processed by the filter unit 260may be a part of a reference image. That is, a reference image is areconstructed image composed of reconstructed blocks processed by thefilter unit 260. The stored reference image may be used later in interprediction or motion compensation.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image. FIG. 3 schematically shows anexample of partitioning a single unit into a plurality of lower units.

In order to efficiently partition an image, when encoding and decoding,a coding unit (CU) may be used. The coding unit may be used as a basicunit when encoding/decoding the image. In addition, the coding unit maybe used as a unit for distinguishing an intra prediction mode and aninter prediction mode when encoding/decoding the image. The coding unitmay be a basic unit used for prediction, transform, quantization,inverse-transform, dequantization, or an encoding/decoding process of atransform coefficient.

Referring to FIG. 3 , an image 300 is sequentially partitioned in alargest coding unit (LCU), and a LCU unit is determined as a partitionstructure. Herein, the LCU may be used in the same meaning as a codingtree unit (CTU). A unit partitioning may mean partitioning a blockassociated with to the unit. In block partition information, informationof a unit depth may be included. Depth information may represent anumber of times or a degree or both in which a unit is partitioned. Asingle unit may be partitioned into a plurality of lower level unitshierarchically associated with depth information based on a treestructure. In other words, a unit and a lower level unit generated bypartitioning the unit may correspond to a node and a child node of thenode, respectively. Each of partitioned lower unit may have depthinformation. Depth information may be information representing a size ofa CU, and may be stored in each CU. Unit depth represents times and/ordegrees related to partitioning a unit. Therefore, partitioninginformation of a lower-level unit may comprise information on a size ofthe lower-level unit.

A partition structure may mean a distribution of a coding unit (CU)within a CTU 310. Such a distribution may be determined according towhether or not to partition a single CU into a plurality (positiveinteger equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs.A horizontal size and a vertical size of the CU generated bypartitioning may respectively be half of a horizontal size and avertical size of the CU before partitioning, or may respectively havesizes smaller than a horizontal size and a vertical size beforepartitioning according to a number of times of partitioning. The CU maybe recursively partitioned into a plurality of CUs. By the recursivepartitioning, at least one among a height and a width of a CU afterpartitioning may decrease comparing with at least one among a height anda width of a CU before partitioning. Partitioning of the CU may berecursively performed until to a predefined depth or predefined size.For example, a depth of a CTU may be 0, and a depth of a smallest codingunit (SCU) may be a predefined maximum depth. Herein, the CTU may be acoding unit having a maximum coding unit size, and the SCU may be acoding unit having a minimum coding unit size as described above.Partitioning is started from the CTU 310, a CU depth increases by 1 as ahorizontal size or a vertical size or both of the CU decreases bypartitioning. For example, for each depth, a CU which is not partitionedmay have a size of 2N×2N. Also, in case of a CU which is partitioned, aCU with a size of 2N×2N may be partitioned into four CUs with a size ofN×N. A size of N may decrease to half as a depth increase by 1.

In addition, information whether or not the CU is partitioned may berepresented by using partition information of the CU. The partitioninformation may be 1-bit information. All CUs, except for a SCU, mayinclude partition information. For example, when a value of partitioninformation is a first value, the CU may not be partitioned, when avalue of partition information is a second value, the CU may bepartitioned.

Referring to FIG. 3 , a CTU having a depth 0 may be a 64×64 block. 0 maybe a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may bea maximum depth. A CU of a 32×32 block and a 16×16 block may berespectively represented as a depth 1 and a depth 2.

For example, when a single coding unit is partitioned into four codingunits, a horizontal size and a vertical size of the four partitionedcoding units may be a half size of a horizontal and vertical size of theCU before being partitioned. In one embodiment, when a coding unithaving a 32×32 size is partitioned into four coding units, each of thefour partitioned coding units may have a 16×16 size. When a singlecoding unit is partitioned into four coding units, it may be called thatthe coding unit may be partitioned (quad-tree partitioned) into aquad-tree form.

For example, when a single coding unit is partitioned into two codingunits, a horizontal or vertical size of the two coding units may be ahalf of a horizontal or vertical size of the coding unit before beingpartitioned. For example, when a coding unit having a 32×32 size ispartitioned in a vertical direction, each of two partitioned codingunits may have a size of 16×32. For example, when a coding unit having asize of 8×32 is horizontally partitioned into two sub-coding units, eachof the two sub-coding units may have a size of 8×16. When a singlecoding unit is partitioned into two coding units, it may be called thatthe coding unit is partitioned (binary-tree partitioned) in abinary-tree form.

For example, when one coding unit is partitioned into three sub-codingunits, the horizontal or vertical size of the coding unit can bepartitioned with a ratio of 1:2:1, thereby producing three sub-codingunits whose horizontal or vertical sizes are in a ratio of 1:2:1. Forexample, when a coding unit having a size of 16×32 is horizontallypartitioned into three sub-coding units, the three sub-coding units mayhave sizes of 16×8, 16×16, and 16×8 respectively, in the order from theuppermost to the lowermost sub-coding unit. For example, when a codingunit having a size of 32×32 is vertically split into three sub-codingunits, the three sub-coding units may have sizes of 8×32, 16×32, and8×32, respectively in the order from the left to the right sub-codingunit. When one coding unit is partitioned into three sub-coding units,it can be said that the coding unit is ternary-tree partitioned orpartitioned by a ternary tree partition structure.

In FIG. 3 , a coding tree unit (CTU) 320 is an example of a CTU to whicha quad tree partition structure, a binary tree partition structure, anda ternary tree partition structure are all applied.

As described above, in order to partition the CTU, at least one of aquad tree partition structure, a binary tree partition structure, and aternary tree partition structure may be applied. Various tree partitionstructures may be sequentially applied to the CTU, according to apredetermined priority order. For example, the quad tree partitionstructure may be preferentially applied to the CTU. A coding unit thatcannot be partitioned any longer using a quad tree partition structuremay correspond to a leaf node of a quad tree. A coding unitcorresponding to a leaf node of a quad tree may serve as a root node ofa binary and/or ternary tree partition structure. That is, a coding unitcorresponding to a leaf node of a quad tree may be further partitionedby a binary tree partition structure or a ternary tree partitionstructure, or may not be further partitioned. Therefore, by preventing acoding block that results from binary tree partitioning or ternary treepartitioning of a coding unit corresponding to a leaf node of a quadtree from undergoing further quad tree partitioning, block partitioningand/or signaling of partition information can be effectively performed.

The fact that a coding unit corresponding to a node of a quad tree ispartitioned may be signaled using quad partition information. The quadpartition information having a first value (e.g., “1”) may indicate thata current coding unit is partitioned by the quad tree partitionstructure. The quad partition information having a second value (e.g.,“0”) may indicate that a current coding unit is not partitioned by thequad tree partition structure. The quad partition information may be aflag having a predetermined length (e.g., one bit).

There may not be a priority between the binary tree partitioning and theternary tree partitioning. That is, a coding unit corresponding to aleaf node of a quad tree may further undergo arbitrary partitioningamong the binary tree partitioning and the ternary tree partitioning. Inaddition, a coding unit generated through the binary tree partitioningor the ternary tree partitioning may undergo a further binary treepartitioning or a further ternary tree partitioning, or may not befurther partitioned.

A tree structure in which there is no priority among the binary treepartitioning and the ternary tree partitioning is referred to as amulti-type tree structure. A coding unit corresponding to a leaf node ofa quad tree may serve as a root node of a multi-type tree. Whether topartition a coding unit which corresponds to a node of a multi-type treemay be signaled using at least one of multi-type tree partitionindication information, partition direction information, and partitiontree information. For partitioning of a coding unit corresponding to anode of a multi-type tree, the multi-type tree partition indicationinformation, the partition direction information, and the partition treeinformation may be sequentially signaled.

The multi-type tree partition indication information having a firstvalue (e.g., “1”) may indicate that a current coding unit is to undergoa multi-type tree partitioning. The multi-type tree partition indicationinformation having a second value (e.g., “0”) may indicate that acurrent coding unit is not to undergo a multi-type tree partitioning.

When a coding unit corresponding to a node of a multi-type tree ispartitioned by a multi-type tree partition structure, the coding unitmay further include partition direction information. The partitiondirection information may indicate in which direction a current codingunit is to be partitioned for the multi-type tree partitioning. Thepartition direction information having a first value (e.g., “1”) mayindicate that a current coding unit is to be vertically partitioned. Thepartition direction information having a second value (e.g., “0”) mayindicate that a current coding unit is to be horizontally partitioned.

When a coding unit corresponding to a node of a multi-type tree ispartitioned by a multi-type tree partition structure, the current codingunit may further include partition tree information. The partition treeinformation may indicate a tree partition structure which is to be usedfor partitioning of a node of a multi-type tree. The partition treeinformation having a first value (e.g., “1”) may indicate that a currentcoding unit is to be partitioned by a binary tree partition structure.The partition tree information having a second value (e.g., “0”) mayindicate that a current coding unit is to be partitioned by a ternarytree partition structure.

The partition indication information, the partition tree information,and the partition direction information may each be a flag having apredetermined length (e.g., one bit).

At least any one of the quad-tree partition indication information, themulti-type tree partition indication information, the partitiondirection information, and the partition tree information may be entropyencoded/decoded. For the entropy-encoding/decoding of those types ofinformation, information on a neighboring coding unit adjacent to thecurrent coding unit may be used. For example, there is a highprobability that the partition type (the partitioned or non-partitioned,the partition tree, and/or the partition direction) of a leftneighboring coding unit and/or an upper neighboring coding unit of acurrent coding unit is similar to that of the current coding unit.Therefore, context information for entropy encoding/decoding of theinformation on the current coding unit may be derived from theinformation on the neighboring coding units. The information on theneighboring coding units may include at least any one of quad partitioninformation, multi-type tree partition indication information, partitiondirection information, and partition tree information.

As another example, among binary tree partitioning and ternary treepartitioning, binary tree partitioning may be preferentially performed.That is, a current coding unit may primarily undergo binary treepartitioning, and then a coding unit corresponding to a leaf node of abinary tree may be set as a root node for ternary tree partitioning. Inthis case, neither quad tree partitioning nor binary tree partitioningmay not be performed on the coding unit corresponding to a node of aternary tree.

A coding unit that cannot be partitioned by a quad tree partitionstructure, a binary tree partition structure, and/or a ternary treepartition structure becomes a basic unit for coding, prediction and/ortransformation. That is, the coding unit cannot be further partitionedfor prediction and/or transformation. Therefore, the partition structureinformation and the partition information used for partitioning a codingunit into prediction units and/or transformation units may not bepresent in a bitstream.

However, when the size of a coding unit (i.e., a basic unit forpartitioning) is larger than the size of a maximum transformation block,the coding unit may be recursively partitioned until the size of thecoding unit is reduced to be equal to or smaller than the size of themaximum transformation block. For example, when the size of a codingunit is 64×64 and when the size of a maximum transformation block is32×32, the coding unit may be partitioned into four 32×32 blocks fortransformation. For example, when the size of a coding unit is 32×64 andthe size of a maximum transformation block is 32×32, the coding unit maybe partitioned into two 32×32 blocks for the transformation. In thiscase, the partitioning of the coding unit for transformation is notsignaled separately, and may be determined through comparison betweenthe horizontal or vertical size of the coding unit and the horizontal orvertical size of the maximum transformation block. For example, when thehorizontal size (width) of the coding unit is larger than the horizontalsize (width) of the maximum transformation block, the coding unit may bevertically bisected. For example, when the vertical size (length) of thecoding unit is larger than the vertical size (length) of the maximumtransformation block, the coding unit may be horizontally bisected.

Information of the maximum and/or minimum size of the coding unit andinformation of the maximum and/or minimum size of the transformationblock may be signaled or determined at an upper level of the codingunit. The upper level may be, for example, a sequence level, a picturelevel, a slice level, or the like. For example, the minimum size of thecoding unit may be determined to be 4×4. For example, the maximum sizeof the transformation block may be determined to be 64×64. For example,the minimum size of the transformation block may be determined to be4×4.

Information of the minimum size (quad tree minimum size) of a codingunit corresponding to a leaf node of a quad tree and/or information ofthe maximum depth (the maximum tree depth of a multi-type tree) from aroot node to a leaf node of the multi-type tree may be signaled ordetermined at an upper level of the coding unit. For example, the upperlevel may be a sequence level, a picture level, a slice level, or thelike. Information of the minimum size of a quad tree and/or informationof the maximum depth of a multi-type tree may be signaled or determinedfor each of an intra slice and an inter slice.

Difference information between the size of a CTU and the maximum size ofa transformation block may be signaled or determined at an upper levelof the coding unit. For example, the upper level may be a sequencelevel, a picture level, a slice level, or the like. Information of themaximum size of the coding units corresponding to the respective nodesof a binary tree (hereinafter, referred to as a maximum size of a binarytree) may be determined based on the size of the coding tree unit andthe difference information. The maximum size of the coding unitscorresponding to the respective nodes of a ternary tree (hereinafter,referred to as a maximum size of a ternary tree) may vary depending onthe type of slice. For example, for an intra slice, the maximum size ofa ternary tree may be 32×32. For example, for an inter slice, themaximum size of a ternary tree may be 128×128. For example, the minimumsize of the coding units corresponding to the respective nodes of abinary tree (hereinafter, referred to as a minimum size of a binarytree) and/or the minimum size of the coding units corresponding to therespective nodes of a ternary tree (hereinafter, referred to as aminimum size of a ternary tree) may be set as the minimum size of acoding block.

As another example, the maximum size of a binary tree and/or the maximumsize of a ternary tree may be signaled or determined at the slice level.Alternatively, the minimum size of the binary tree and/or the minimumsize of the ternary tree may be signaled or determined at the slicelevel.

Depending on size and depth information of the above-described variousblocks, quad partition information, multi-type tree partition indicationinformation, partition tree information and/or partition directioninformation may be included or may not be included in a bit stream.

For example, when the size of the coding unit is not larger than theminimum size of a quad tree, the coding unit does not contain quadpartition information. Thus, the quad partition information may bededuced from a second value.

For example, when the sizes (horizontal and vertical sizes) of a codingunit corresponding to a node of a multi-type tree are larger than themaximum sizes (horizontal and vertical sizes) of a binary tree and/orthe maximum sizes (horizontal and vertical sizes) of a ternary tree, thecoding unit may not be binary-tree partitioned or ternary-treepartitioned. Accordingly, the multi-type tree partition indicationinformation may not be signaled but may be deduced from a second value.

Alternatively, when the sizes (horizontal and vertical sizes) of acoding unit corresponding to a node of a multi-type tree are the same asthe maximum sizes (horizontal and vertical sizes) of a binary treeand/or are two times as large as the maximum sizes (horizontal andvertical sizes) of a ternary tree, the coding unit may not be furtherbinary-tree partitioned or ternary-tree partitioned. Accordingly, themulti-type tree partition indication information may not be signaled butbe derived from a second value. This is because when a coding unit ispartitioned by a binary tree partition structure and/or a ternary treepartition structure, a coding unit smaller than the minimum size of abinary tree and/or the minimum size of a ternary tree is generated.

Alternatively, when the depth of a coding unit corresponding to a nodeof a multi-type tree is equal to the maximum depth of the multi-typetree, the coding unit may not be further binary-tree partitioned and/orternary-tree partitioned. Accordingly, the multi-type tree partitionindication information may not be signaled but may be deduced from asecond value.

Alternatively, only when at least one of vertical direction binary treepartitioning, horizontal direction binary tree partitioning, verticaldirection ternary tree partitioning, and horizontal direction ternarytree partitioning is possible for a coding unit corresponding to a nodeof a multi-type tree, the multi-type tree partition indicationinformation may be signaled. Otherwise, the coding unit may not bebinary-tree partitioned and/or ternary-tree partitioned. Accordingly,the multi-type tree partition indication information may not be signaledbut may be deduced from a second value.

Alternatively, only when both of the vertical direction binary treepartitioning and the horizontal direction binary tree partitioning orboth of the vertical direction ternary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingunit corresponding to a node of a multi-type tree, the partitiondirection information may be signaled. Otherwise, the partitiondirection information may not be signaled but may be derived from avalue indicating possible partitioning directions.

Alternatively, only when both of the vertical direction binary treepartitioning and the vertical direction ternary tree partitioning orboth of the horizontal direction binary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingtree corresponding to a node of a multi-type tree, the partition treeinformation may be signaled. Otherwise, the partition tree informationmay not be signaled but be deduced from a value indicating a possiblepartitioning tree structure.

FIG. 4 is a view showing an intra-prediction process.

Arrows from center to outside in FIG. 4 may represent predictiondirections of intra prediction modes.

Intra encoding and/or decoding may be performed by using a referencesample of a neighbor block of the current block. A neighbor block may bea reconstructed neighbor block. For example, intra encoding and/ordecoding may be performed by using an encoding parameter or a value of areference sample included in a reconstructed neighbor block.

A prediction block may mean a block generated by performing intraprediction. A prediction block may correspond to at least one among CU,PU and TU. A unit of a prediction block may have a size of one among CU,PU and TU. A prediction block may be a square block having a size of2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

Intra prediction may be performed according to intra prediction mode forthe current block. The number of intra prediction modes which thecurrent block may have may be a fixed value and may be a valuedetermined differently according to an attribute of a prediction block.For example, an attribute of a prediction block may comprise a size of aprediction block and a shape of a prediction block, etc.

The number of intra-prediction modes may be fixed to N regardless of ablock size. Or, the number of intra prediction modes may be 3, 5, 9, 17,34, 35, 36, 65, 67 or 131 etc. Alternatively, the number ofintra-prediction modes may vary according to a block size or a colorcomponent type or both. For example, the number of intra predictionmodes may vary according to whether the color component is a luma signalor a chroma signal. For example, as a block size becomes large, a numberof intra-prediction modes may increase. Alternatively, a number ofintra-prediction modes of a luma component block may be larger than anumber of intra-prediction modes of a chroma component block.

An intra-prediction mode may be a non-angular mode or an angular mode.The non-angular mode may be a DC mode or a planar mode, and the angularmode may be a prediction mode having a specific direction or angle. Theintra-prediction mode may be expressed by at least one of a mode number,a mode value, a mode numeral, a mode angle, and mode direction. A numberof intra-prediction modes may be M, which is larger than 1, includingthe non-angular and the angular mode.

In order to intra-predict a current block, a step of determining whetheror not samples included in a reconstructed neighbor block may be used asreference samples of the current block may be performed. When a samplethat is not usable as a reference sample of the current block ispresent, a value obtained by duplicating or performing interpolation onat least one sample value among samples included in the reconstructedneighbor block or both may be used to replace with a non-usable samplevalue of a sample, thus the replaced sample value is used as a referencesample of the current block.

When intra-predicting, a filter may be applied to at least one of areference sample and a prediction sample based on an intra-predictionmode and a current block size.

In case of a planar mode, when generating a prediction block of acurrent block, according to a position of a prediction target samplewithin a prediction block, a sample value of the prediction targetsample may be generated by using a weighted sum of an upper and leftside reference sample of a current sample, and a right upper side andleft lower side reference sample of the current block. In addition, incase of a DC mode, when generating a prediction block of a currentblock, an average value of upper side and left side reference samples ofthe current block may be used. In addition, in case of an angular mode,a prediction block may be generated by using an upper side, a left side,a right upper side, and/or a left lower side reference sample of thecurrent block. In order to generate a prediction sample value,interpolation of a real number unit may be performed.

An intra-prediction mode of a current block may be entropyencoded/decoded by predicting an intra-prediction mode of a blockpresent adjacent to the current block. When intra-prediction modes ofthe current block and the neighbor block are identical, information thatthe intra-prediction modes of the current block and the neighbor blockare identical may be signaled by using predetermined flag information.In addition, indicator information of an intra-prediction mode that isidentical to the intra-prediction mode of the current block amongintra-prediction modes of a plurality of neighbor blocks may besignaled. When intra-prediction modes of the current block and theneighbor block are different, intra-prediction mode information of thecurrent block may be entropy encoded/decoded by performing entropyencoding/decoding based on the intra-prediction mode of the neighborblock.

FIG. 5 is a view showing intra-prediction according to the presentinvention.

Intra-prediction of a current block may include: step S510 of derivingan intra-prediction mode, step S520 of configuring a reference sample,and/or step S530 of performing intra-prediction.

In step S510, an intra-prediction mode of a current block may bederived. The intra-prediction mode of the current block may be derivedby using a method of using an intra-prediction mode of a neighbor block,a method of entropy encoding/decoding an intra-prediction mode of acurrent block from a bitstream, a method of using a coding parameter ofa neighbor block or a method of using intra prediction mode of a colorcomponent. According to the method of using the intra-prediction mode ofthe neighbor block, the intra-prediction mode of the current block maybe derived by using at least one intra-prediction mode derived by usingan intra-prediction mode of a neighbor block, a combination of at leastone intra-prediction mode of a neighbor block, and at least one MPM.

In step S520, a reference sample may be configured by performing atleast one of reference sample selecting, reference sample padding andreference sample filtering.

In step S530, intra-prediction may be performed by performing at leastone of non-angular prediction, angular prediction, positionalinformation based prediction, and inter color component prediction. Whenangular prediction is performed, prediction having angles orinterpolation filter types different by a predetermined unit thatincludes at least one sample of the current block may be performed. Thepredetermined unit may be, for example, at least one of a singularsample, a sample group, a line, and a block. In step S530, filtering ona prediction sample may be additionally performed. The filter type maymean at least one among a filter tab, a filter coefficient and a filtershape.

An intra prediction may be performed adaptively based on at least oneamong an intra prediction mode, a size of the current block, a form ofthe current block and a location of a prediction sample. For example,whether to use a plurality of reference sample lines, whether to performinterpolation filtering, a coefficient of an interpolation filter,whether to perform filtering, whether to perform weighted averagingand/or a weight used for weighted averaging may be adaptively determinedbased on at least one among an intra prediction mode, a size of thecurrent block, a form of the current block and a location of aprediction sample. The specification for the above will be explainedlater.

In order to derive the intra-prediction mode of the current block, atleast one reconstructed neighbor block may be used. A position of thereconstructed neighbor block may be a fixed position that is predefined,or may be a position derived by encoding/decoding. Hereinafter,encoding/decoding may mean entropy encoding and decoding. For example,when a coordinate of a left upper corner side sample of a current blockhaving a W×H size is (0, 0), a neighbor block may be at least one ofblocks adjacent to coordinate of (−1, H−1), (W−1, −1), (W, −1), (−1, H),and (−1, −1), and neighbor blocks of the above blocks. Here, W and H mayrepresent length or the number of samples of width (W) and height (H) ofthe current block.

An intra-prediction mode of a neighbor block which is not available maybe replaced with a predetermined intra-prediction mode. Thepredetermined intra-prediction mode may be, for example, a DC mode, aplanar mode, a vertical mode, a horizontal mode, and/or a diagonal mode.For example, when a neighbor block is positioned outside of a boundaryof at least one predetermined unit of a picture, a slice, a tile, and acoding tree unit, the neighbor block is inter-predicted, or when theneighbor block is encoded in a PCM mode, the corresponding block may bedetermined as non-available. Alternatively, when the neighbor block isunavailable, the intra prediction mode of the corresponding block is notreplaced and not used.

The intra-prediction mode of the current block may be derived as astatistical value of an intra-prediction mode of a predeterminedpositional neighbor block or an intra-prediction mode of at least twoneighbor blocks. In the present description, the statistical value maymean at least one of an average value, a maximum value, a minimum value,a mode, a median value, a weighted average value, and an interpolationvalue.

Alternatively, the intra-prediction mode of the current block may bederived based on a size of neighbor blocks. For example, anintra-prediction mode of a neighbor block having relatively large sizemay be derived as the intra-prediction mode of the current block.Alternatively, a statistical value may be calculated by assigning alarge weight on an intra-prediction mode of a block having relativelylarge size. Alternatively, a mode to which a relatively large weight isassigned may be pre-defined or signaled. For example, a relatively largeweight may be assigned to at least one among a vertical directionalmode, a horizontal directional mode, a diagonal directional mode andnon-directional mode. The same weight may be assigned to the abovemodes.

Alternatively, whether or not the intra-prediction mode of the neighborblock is angular mode may be considered. For example, when theintra-prediction mode of the neighbor block is a non-angular mode, thenon-angular mode may be derived as the intra-prediction mode of thecurrent block. Alternatively, an intra-prediction mode of other neighborblock, except for the non-angular mode, may be derived as theintra-prediction mode of the current block.

In order to derive the intra-prediction mode of the current block, oneor more most probable mode (MPM) lists may be configured by using anintra-prediction mode of a neighbor block. A number N of candidate modesincluded in an MPM list may be fixed, or may be determined according toa size or form or both of the current block. The MPM list may beconfigured not to include an overlapped mode. When a number of availablecandidate modes is smaller than N, a predetermined candidate mode amongavailable candidate modes, for example, a mode obtained by adding orsubtracting a predetermined offset to an angular mode may be added tothe one or more MPM lists. Alternatively, at least one of a horizontalmode, a vertical mode, a 45 angular mode, a 135 angular mode, a 225angular mode, and a non-angular mode may be added to the MPM list. Thepredetermined offset may be 1, 2, 3, 4, or a positive integer.

The MPM list may be configured in a predetermined sequence based on aposition of the neighbor block. For example, the predetermined sequencemay be a sequence of blocks adjacent to a left side, an upper side, aleft lower corner side, a right upper corner side, and a left uppercorner side of the current block. A non-angular mode may be included inthe MPM list at an arbitrary position. For example, it may be added nextto intra-prediction modes of blocks adjacent to a left side and an upperside.

An MPM list generated based on the current block may be used as an MPMlist for at least one sub block included in the current block. An orderbetween candidate modes configuring an MPM list, the number of candidatemodes included in an MPM list, etc, may be determined based on a size, aform and/or a component of the current block.

Alternatively, a group of modes may be configured by selecting a part ofmodes among modes which are not included in an MPM list. The configuredgroup of modes may be utilized as another list. For example, a group ofmodes may be configured using modes which are obtained by sampling witha predetermined interval after arranging modes which are not MPMcandidates, or using modes which are obtained by adding/subtracting n (nis an integer equal to or larger than 1) to/from a MPM candidate mode.

As another embodiment, the intra-prediction mode of the current blockmay be derived by using an intra-prediction mode derived by using an MPMlist and an intra-prediction mode of a neighbor block. For example, whenthe intra-prediction mode derived by using the MPM list is Pred_mpm, thePred_mpm may be changed by using the intra-prediction mode of theneighbor block. For example, when Pred_mpm is larger than theintra-prediction mode of the neighbor block (or larger than astatistical value of at least two intra-prediction modes), Pred_mpm maybe increased by n, otherwise, Pred_mpm may be decreased by n. Herein, nmay be a predetermined integer such as +1, +2, +3, 0, −1, −2, −3, etc.The intra-prediction mode of the current block may be derived as thechanged Pred_mpm. Alternatively, when at least one of Pred_mpm andintra-prediction modes of the neighbor block is a non-angular mode, theintra-prediction mode of the current block may be derived as thenon-angular mode. Alternatively, the intra-prediction mode of thecurrent block may be derived as an angular mode.

According to a further embodiment of the present invention relating to amethod of deriving an intra prediction mode, an intra prediction mode ofa current block may be derived by using an intra prediction mode of adifferent color component. For example, when the current block is achroma block, an intra prediction mode of a luma block corresponding tothe chroma block can be used to derive an intra prediction mode of thechroma block. As the luma block corresponding to the chroma block, theremay be one or more luma blocks. The corresponding luma block may bedetermined depending on at least any one of the size, the shape, and theencoding parameter of a chroma block. Alternatively, the correspondingluma block may be determined depending on at least any one of the size,the shape, and the encoding parameter of a luma block.

The luma block corresponding to the chroma block may be composed of aplurality of partitions. All or part of the plurality of partitions mayhave different intra prediction modes thereof. An intra prediction modeof the chroma block may be derived on the basis of all or part of theplurality of partitions included in the corresponding luma block. Inthis case, some partitions may be selectively used, in which the usedpartitions are selected based on the comparison of the block size, theshape, the depth information, etc. of the chroma block with those of theluma block (all or part of the plurality of partitions). A partition ata position in the luma block corresponding to a predetermined positionin the chroma block may be selectively used. The predetermined positionmay refer to a corner sample (e.g., upper left sample) position in thechroma block or a center sample position in the chroma block.

The method of deriving an intra prediction mode of one color componentblock using an intra prediction mode of a different color componentblock (i.e. inter color component intra prediction mode) according tothe present invention is not limited to the example in which an intraprediction mode of a luma block corresponding to a chroma block is used.For example, an intra prediction mode of a chroma block may be derivedby using or sharing at least any one of an MPM index mpm_idx and an MPMlist of a luma block corresponding to the chroma block.

FIG. 6 is an exemplary diagram illustrating the relationship between aluma block and a chroma block.

In the example illustrated in FIG. 6 , a sample ratio of colorcomponents is 4:2:0, and at least one of luma blocks A, B, C, and Dcorresponds to one chroma block.

With reference to FIG. 6 , an intra prediction mode of one chroma blockmay be derived by using an intra prediction mode of the luma block Acorresponding to a sample at an upper left position (0,0) in the chromablock or an intra prediction mode of the luma block D corresponding to asample at a center position (nS/2, nS/2) in the chroma block. Thepredetermined position in the chroma block is not limited to the upperleft position (0, 0) or the center position (nS/2, nS/2). For example,The predetermined position may be an upper right position, a lower leftposition, and/or a lower right position.

The predetermined position may be selected on the basis of the shape ofthe chroma block. For example, with the chroma block having a squareshape, the predetermined position may be a center sample position. Withthe chroma block having an oblong shape, the predetermined position maybe an upper left sample position. Alternatively, the predeterminedposition may be a position of an upper left sample in the chroma blockhaving a square shape or a position of a center sample in the chromablock having an oblong shape.

According to a further embodiment, an intra prediction mode of a chromablock may be derived by using statistic figures of one or more intraprediction modes of a luma block having an equal size to the chromablock.

In the example illustrated in FIG. 6 , a mode corresponding to theaverage of the intra prediction modes of the luma blocks A and D or amode corresponding to the average of the intra prediction modes of theblocks A, B, C, and D inside the luma block corresponding to a size ofthe chroma block is derived as the intra prediction mode of the chromablock.

When there are multiple intra prediction modes of available luma blocks,all or part of them may be selected. The selection is performed based onthe predetermined position in the chroma block or based on the size(s),the shape(s), and/or the depth(s) of the chroma block, the luma block,or both. The intra prediction mode of the chroma block can be derived byusing the selected intra prediction mode of the luma block.

For example, the size of the luma block A corresponding to the upperleft sample position (0,0) in the chroma block and the size of theluminance bock D corresponding to the center sample position (nS/2,nS/2) in the chroma block are compared, and the intra prediction mode ofthe luma block D having a larger size may be used to derive the intraprediction mode of the chroma block.

Alternatively, when the size of a luma block corresponding to apredetermined position in a chroma block is equal to or larger than thesize of the chroma block, an intra prediction mode of the chroma blockis derived by using the intra prediction mode of the luma block.

Alternatively, when the size of a chroma block is within a predeterminedrange, an intra prediction mode of the chroma block is derived by usingan intra prediction mode of a luma block corresponding to the upper leftsample position (0, 0) in the chroma block.

Alternatively, when the size of a chroma block is within a predeterminedrange, the size of a luma block corresponding to a predeterminedposition (0, 0) of the chroma block and the size of a luma blockdisposed at another predetermined position (nS/2, nS/2) of the chromablock are compared, and an intra prediction mode of the chroma block isderived by using the intra prediction mode of the luma block having alarger size.

The predetermined range may be derived from at least any one piece ofinformation among information signaled through a bitstream, informationof the size (and/or depth) of a block (a chroma block, a luma block, orboth), and information predefined in an encoder or a decoder.

Alternatively, when a chroma block has an oblong shape, an intraprediction mode of the chroma block may be derived by using an intraprediction mode of a luma block corresponding to a center sampleposition (nS/2, nS/2) in the chroma block.

Among the plurality of partitions of the luma block, a partition havingthe same shape as the chroma block may be used. For example, when thechroma block has a square shape or a non-square shape, a partitionhaving a square shape or a non-square shape, selected among theplurality of partitions of the luma block, may be used.

In the example described with reference to FIG. 6 , the method ofderiving an intra prediction mode of a chroma block using an intraprediction mode of a luma block also applies to a case in which an intraprediction mode of a luma block is used as an intra prediction mode of achroma block as it is. The method of deriving an intra prediction modeof a chroma block is not limited to the method of using an intraprediction mode of the corresponding luma block. For example, an intraprediction mode of a chroma block can be derived from information,including an MPM list and an MPM index mpm_idx, which is used to derivean intra prediction mode of a luma block.

Alternatively, the MPM list of the chroma block can be constructed usingthe intra prediction mode of the luma block corresponding to the sampleof the predetermined position in the chroma block. In this case, thempm-idx information of the chroma block may be encoded and signaled. TheMPM list of the chroma block may be constructed in a similar way to theconstruction of the MPM list of the luma block. MPM candidates of thechroma block may include intra prediction modes of neighbor chromablocks and/or intra prediction modes of luma blocks corresponding to thechroma block.

When an MPM flag is 0, a second MPM list including at least oneintra-prediction mode may be configured, and the intra-prediction modeof the current block may be derived by using a second MPM index(2nd_mpm_idx). Herein, a second indicator (for example, a second MPMflag) indicating whether or not the intra-prediction mode of the currentblock is included in the second MPM list may be encoded/decoded. Similarto a first MPM list, the second MPM list may be configured by usingintra-prediction modes of the neighbor block. Herein, theintra-prediction mode included in the first MPM list may not be includedin the second MPM list. A number of MPM lists is not limited to 1 or 2,N MPM lists may be used.

When the intra-prediction mode of the current block is not included inone of a plurality of MPM lists, a luma component intra-prediction modeof the current block may be encoded/decoded. In addition, a chromacomponent intra-prediction mode may be derived and encoded/decoded basedon an associated luma component intra-prediction mode.

When the current block is partitioned into a plurality of sub-blocks, inorder to derive an intra-prediction mode of each sub-block, at least oneof the described methods may be applied.

A size or form or both of a sub-block may be a predetermined size orblock or both (for example, 4×4), or may be determined according to asize or form or both of the current block. Alternatively, the size ofthe sub-block may be determined based on whether or not a neighbor blockof the current block is partitioned, or may be determined based on anintra-prediction mode of a neighbor block of the current block. Forexample, the current block may be partitioned based on a boundary atwhich an intra-prediction mode of a neighbor block is different.Alternatively, the current block may be partitioned based on whether theneighbor block is an intra coding block or an inter coding block.

An indicator (for example, NDIP flag) representing that theintra-prediction mode of the current block is derived by using theintra-prediction mode of the neighbor block may be encoded/decoded. Theindicator may be encoded/decoded by at least one unit of the currentblock and the sub-block. Herein, when a size of the current block or thesub-block corresponds to a predetermined size or a predetermined sizerange, the indicator may be encoded/decoded.

Determining whether or not the size of the current block corresponds toa predetermined size may be performed based on a horizontal or verticallength of the current block. For example, when the horizontal orvertical length is a length capable of being partitioned, it isdetermined that the size of the current block corresponds to apredetermined size.

When the current block is partitioned into a plurality of sub-blocks, anintra-prediction mode of the plurality of sub-blocks may be derived in azig-zag sequence, or may be derived in parallel. An intra-predictionmode of the sub-block may be derived by at least one of methods ofderiving the intra-prediction mode of the current block. Herein, theneighbor block of the current block may be used as a neighbor block ofeach sub-block. Alternatively, the sub-block within the current blockmay be used as a neighbor block of each sub-block.

An intra-prediction mode for the first sub-block among sub-blocks insidethe current block may be derived with a different method from othersub-blocks. The first sub-block, for example, may be the first sub-blockin a scan order.

An intra-prediction mode of a sub-block included in a current block maybe derived by using an average value of an intra-prediction mode of thecurrent block and an intra-prediction mode of a block adjacent to a leftand upper side of a sample positioned at (0, 0) of each sub-block. Forexample, when an intra-prediction mode of a current block is larger thanthe above average value, the half of the above average value may besubtracted from the derived intra-prediction mode. When theintra-prediction mode of the current block is equal to or less than theabove average value, the half of the above average value may be added tothe derived intra-prediction mode.

Intra-prediction information may be signaled through at least one of avideo parameter set (VPS), a sequence parameter set (SPS), a pictureparameter set (PPS), an adaptation parameter set (APS), a slice header,and a tile header. In a predetermined block size or less, at least onepiece of intra-prediction information may not be signaled. Herein,intra-prediction information of a previously encoded/decoded block (forexample, higher block) may be used.

A reference sample for intra-prediction may be configured based on thederived intra-prediction mode. In the description hereinafter, a currentblock may mean a prediction block or a sub-block having a size/formsmaller than a size/form of the prediction block. The reference samplemay be configured by using at least one sample reconstructed adjacent toa current block or by using a combination of samples. In addition,filtering may be applied to the configured reference sample.

A number or position or both of reconstructed sample lines used forconfiguring the reference sample may vary according to a position of acurrent block within a coding tree block. Each reconstructed sample on aplurality of reconstructed sample lines may be used as a referencesample at it is. Alternatively, a predetermined filter may be applied tothe reconstructed sample, and a reference sample may be generated byusing the filtered reconstructed sample. Reconstructed samples to whicha filter is applied may be included in the same reconstructed sampleline or in different reconstructed sample lines.

The configured reference sample may be represented as ref[m, n], and asample obtained by applying a filter to the configured reference samplemay be represented as rec[m, n]. Herein, m or n may be a predeterminedinteger value representing a position of a sample. When a position of aleft upper side sample within the current block is (0, 0), a position ofa left upper side reference sample of the current block may be set to(−1, −1).

FIG. 7 is a diagram for describing a plurality of reconstructed samplelines.

A reference sample can be constructed by selecting one or morereconstructed sample lines adjacent to the current block. For example,in FIG. 7 , one of the plurality of reconstructed sample lines may beselected so as to construct a reference sample.

For example, a particular reconstructed sample line of the plurality ofreconstructed sample lines may be fixedly or adaptively selected, or anarbitrary reconstructed sample line may be adaptively selected, in orderto construct a reference sample.

In another embodiment, to construct a reference sample, one or morereconstructed sample lines may be selected from the plurality ofreconstructed sample lines illustrated in FIG. 7 , and the selectedreconstructed sample lines may be combined.

For example, as shown in Equation 1, a reference sample may beconstructed using a weighted average of reconstructed samples, in whichthe weights of the reconstructed samples differ according to thedistance between the reconstructed sample and the current block.

ref[−1,−1]=(rec[−2,−1]+2×rec[−1,−1]+rec[−1,−2]+2)>>2

ref[x,−1]=(rec[x,−2]+3×rec[x,−1]+2)>>2,(x=0 to H+W−1)

ref[−1,y]=(rec[−2,y]+3×rec[−1,y]+2)>>2,(y=0 to H+W−1)  [Equation 1]

Alternatively, a reference sample may be constructed using at least oneof a mean value, a maximum value, a minimum value, a median value, and amode value of a plurality of reconstructed samples based on at least oneof the distance from the current block to the correspondingreconstructed sample and the intra prediction mode of the current block.

Alternatively, a reference sample may be constructed based on a change(change amount) between each of the sample values of the successivereconstructed samples. For example, a reference sample may beconstructed based on at least one of a determination of whether thevalues of two successive reconstructed samples differ by more than athreshold value and a determination of whether the values of successivereconstructed samples change continuously or discontinuously. Forexample, when the values of a rec[−1, −1] and a rec[−2, −1] differ bymore than a threshold value, the value of the ref[−1, −1] is determinedas having the value of the rec[−1, −1], or a value corresponding to aweighted average obtained by applying a predetermined weight to thevalue of the rec[−1, −1]. For example, each of the values of thesuccessive reconstructed samples changes by n as the distance betweenthe reconstructed sample and the current block decreases, and thus thevalue of ref[−1, −1] is represented as “ref[−1, −1]=rec[−1, −1]−n”.

In a different embodiment, referring to FIG. 7 , two or morereconstructed sample lines may be selected to construct a referencesample. For example, two lines including a reconstructed sample line 1and a reconstructed sample line 2 may be fixedly selected, or four linesranging from a reconstructed sample line 1 to a reconstructed sampleline 4 may be selected to construct a reference sample.

Alternatively, two or more reconstructed sample lines may be adaptivelyselected to construct a reference sample. For example, one reconstructedsample line may be fixedly selected, and one or more reconstructedsample lines may be adaptively selected among the other reconstructedsample lines to construct a reference sample.

The fixedly selected reconstructed sample line may be predefined in theencoder/decoder. For the case where the fixedly selected reconstructedsample line is predefined, information on the fixedly selectedreconstructed sample line may not be signaled.

The information on the adaptively selected reconstructed sample line(s)may be signaled in the form of an indicator or index. The adaptivelyselected reconstructed sample line may be determined based on at leastone of coding parameters of the current block or a block neighboring thecurrent block. For example, the adaptively selected reconstructed sampleline may be determined based on at least one of the size/shape and intraprediction mode of the current block or the block neighboring thecurrent block. In this case, the information necessary for selection maynot be signaled.

A reference sample line may include one or more samples. For example,the reference sample line may include samples corresponding to a lengthequal to the width (that is, the horizontal dimension) or height (thatis, the vertical dimension) of the current block. As another example,the reference sample line may include samples corresponding to a lengththat is two times the width or height of the current block. As a furtherexample, the reference sample line may include samples corresponding toa length equal to N samples (N is 1, 2, 3, . . . ) plus two times thesum of the width and height of the current block. That is, the referencesample line may include reference samples corresponding to 2×(W+H)+N(where W and H are the width and height of the current block, and N isan integer of 1 or more).

The method of constructing a reference sample adjacent to an upper partof the current block and the method of constructing a reference sampleadjacent to a left part of the current block may differ. For example,the number of reference sample lines located above the current block andthe number of reference sample lines located to the left of the currentblock may differ. For example, the number of reference sample linesadjacent to the upper part of the current block may be one and thenumber of reference sample lines adjacent to the left part of thecurrent block may be two, according to at least one of the width orheight of the current block, and the intra prediction mode of thecurrent block. For example, the length of the reference sample lineabove the current block and the length of the reference sample linelocated to the left of the current block may differ. For example, thelength of the reference sample line may vary according to at least oneof the width or height of the current block and the intra predictionmode of the current block.

Each of the reference sample lines may have a different length. Forexample, referring to FIG. 7 , the lengths of the reconstructed samplelines 2 to 4 may be longer than the reconstructed sample line 1 by alength corresponding to one or more samples.

The length of the reference sample line may be different for each of thereconstructed sample lines. For example, a reconstructed sample line nmay be longer or shorter than a reconstructed sample line n−1 by alength corresponding to m samples. In the example illustrated in FIG. 7, the reconstructed sample line n is longer than the reconstructedsample line n−1 by a length corresponding to one sample.

Alternatively, the reference sample lines may be reconstructed by beingshifted according to the intra prediction mode of the current block. Forexample, when there is no reference sample at the position referenced bya certain intra prediction mode, the reference sample line may beshifted so that the reference sample will be available at the positionreferenced by the intra prediction mode. Which reference sample line isto be shifted, or how far the reference sample line is to be shifted maybe determined based on which intra prediction mode is used for thecurrent block, an angle of a prediction direction, and/or the positionat which the reference sample line is located.

As described above, decision information on whether to construct areference sample using only the nearest reference sample line or using aplurality of reference sample lines may be encoded/decoded. For example,the decision information may be encoded/decoded at the level of at leastone of a sequence, a picture, a slice, a tile, a CTU, a CU, a PU, and aTU. In addition, information on the availability of each of theplurality of reference sample lines may be signaled at a higher level.

At least one of the number, position, and configuration of thereconstructed sample lines used in the reference sample construction maybe differently set when the top boundary or the left boundary of thecurrent block corresponds to the boundary of at least one of a picture,a slice, a tile, and a coding tree block (CTB). For example, when two ormore reference sample lines are constructed, when the top boundary ofthe current block corresponds to the boundary of at least one of apicture, a tile, a slice, and a coding tree block (CTB), one referencesample line adjacent to the upper part of the current block may beconstructed. For example, one reference sample line may be configuredwhen the top boundary of the current block corresponds to the topboundary of a CTU, and otherwise, two or more reference sample lines maybe configured. In this case, since only one reference sample line at thetop boundary of the CTU is used, the size of a line buffer for storingdata of the reference samples of the reference sample line can bereduced.

When selecting a reference sample, availability determination andreference sample padding may be performed for a block containing thereference sample to be used. For example, when a block containing areference sample is available, the corresponding reference sample can beused. On the other hand, when a block containing a reference sample isnot available, the unavailable reference samples in the block may bepadded with one or more available neighboring reference samples.

When a reference sample is located outside the boundary of at least oneof a picture, a tile, a slice, or a coding tree block (CTB), thereference sample may be determined to be unavailable. When the currentblock is coded with constrained intra prediction (CIP), in the casewhere the block including the reference sample has been encoded/decodedin an inter prediction mode, the reference sample is determined to beunavailable.

FIG. 8 is a diagram for describing a process of replacing an unavailablesample with an available sample.

When it is determined that the reconstructed neighboring sample is notavailable, the unavailable sample may be replaced with a reconstructedneighboring sample, which is an available sample. For example, whenthere are both available samples and unavailable samples as illustratedin FIG. 8 , one or more available samples can be used to replace one ormore unavailable samples.

The sample values of the unavailable samples may be replaced with thevalues of the available samples in a predetermined order. The availablesamples used to replace the unavailable samples may be available sampleslocated adjacent to the unavailable samples. When no available sample isadjacent to the unavailable sample, the earliest or closest availablesample may be used to replace the unavailable sample. The replacingorder of the unavailable samples may be, for example, from the bottomleft to the top right. Alternatively, the replacing order may be fromthe top right to the bottom left. Specifically, the replacing order maybe from the top left corner to the top right and/or to the bottom left.Alternatively, the replacing order may be from the top right and/or fromthe bottom left to the top left corner.

For example, filling the unavailable samples with the values ofavailable samples may start from the position 0, which is the bottomleft sample position. That is, the first four unavailable samples may befilled with a value of “a”, and the subsequent 13 unavailable samplesmay be filled with a value of “b”.

For example, the unavailable samples may be filled with a combined valueof the available samples. For example, the unavailable samples may befilled with an average value or an interpolated value of the availablesamples respectively adjacent to both ends of a line of the unavailablesamples. That is, the first four unavailable samples are filled with thevalue “a”, and the next 13 unavailable samples may be filled with theaverage of a value of “b” and a value of “c”, or may be filled byinterpolating the value “b” and the value “c”.

Alternatively, the 13 unavailable samples may be filled with anarbitrary intermediate value between the sample values “b” and “c” ofthe available samples. In this case, the unavailable samples may befilled with different respective values. For example, as the distance ofan unavailable sample to the available sample having the value “a”decreases, the unavailable sample will be filled with a value that iscloser to the value “a”. For example, the closer an unavailable sampleis to an available sample having the value “b”, the closer the valuethat fills the unavailable sample is to the value “b”. That is, thevalue of an unavailable sample may be determined based on the distancebetween the unavailable sample and the available sample having the value“a” or “b”. To replace unavailable samples with available samples, oneor more replacement methods including the methods described above may beadaptively used. The method of replacing unavailable samples withavailable samples may be signaled as information contained in abitstream, or may be predetermined in the encoder/decoder.Alternatively, a replacement method may be derived according to apredetermined determination method. For example, the replacement methodmay be determined based on the difference between the values “a” and “b”or based on the number of unavailable samples. More specifically, thereplacement method may be determined by comparing the difference betweenthe values of two available samples with a threshold value and/or bycomparing the number of unavailable samples with a threshold value. Forexample, when the difference between the values of the two availablesamples is greater than the threshold value, and/or when the number ofunavailable samples is greater than the threshold value, the unavailablesamples may be replaced to have different values from each other. Theselection of the method of replacing unavailable samples with availablesamples may be performed on a per-predetermined-unit basis. For example,the replacement may be selected on a per-video basis, a per-sequencebasis, a per-picture basis, a per-slice basis, a per-tile basis, aper-coding-tree-unit (CTU) basis, a per-coding-unit (CU) basis, aper-prediction-unit (PU) basis, a per-transform-unit (TU) basis, or aper-block basis. At this time, the selection of the method of replacingunavailable samples with available samples may be determined based onthe information signaled on a per-predetermined-unit basis or may bederived on a per-predetermined-unit basis. Alternatively, the selectionmethod for the replacement methods may be predetermined in theencoder/decoder.

When a reference sample is located at a predetermined position, paddingmay be automatically performed without determining whether a blockincluding the reference sample is available or not. For example,referring to FIG. 7 , when the position (x, y) of the top left cornersample of the current block is (0, 0), sample availability may not bedetermined for samples located at (x, y) in which the x coordinate orthe y coordinate is equal to or greater than W+H (x=W+H or greater ory=W+H or greater), and the samples may be padded with neighboringreference samples.

For example, a sample ref[W+H, −2] may be padded with the value of asample ref[W+H−1, −2] without performing the availability determinationon the sample ref[W+H, −2]. As another example, a sample ref[W+H, −3]may be padded with the value of a sample ref[W+H−1, −3] withoutperforming the availability determination on the sample[W+H, −3]. Thatis, the padding may be performed on the samples located at positions (x,y: x is equal to or greater than W+H or y is equal to or greater thanW+H) by using the closest sample on the same sample line withoutperforming the availability determination thereon.

When the position of the top left corner sample of the current block is(0, 0), for samples located at positions (x, y: x is equal to or greaterthan W and is less than W+H) among the samples located above the currentblock, the availability determination will be performed, and then thepadding will be performed according to the result of the availabilitydetermination. For samples located at positions (x, y: y is equal to orgreater than H and is less than W+H) among the samples located to theleft of the current block, the availability determination will beperformed, and the padding will be performed according to theavailability determination.

For example, when the position of the top left corner sample of thecurrent block is (0, 0), for samples corresponding to rec[x, −1] (xranges from −1 to W+H−1) and/or samples corresponding to rec[−1, y] (yranges from 0 to H+W−1), the availability determination and the paddingmay be performed.

For the padding, a plurality of reference sample lines may be used. Forexample, when the padding is performed on a first reference sample lineadjacent to (that is, the closest to) the current block, a secondreference sample line, which is the second closest to the current block,may be used. For example, the padding may be performed according toEquation 2. That is, the sample values of the first reference sampleline may be derived by using the weighted average of samples selectedfrom the first reconstructed reference sample line and samples selectedfrom the second reconstructed reference sample line. In this case, theselected reconstructed sample may be one located at a current sampleposition or at a position adjacent to the current sample position.

ref[x,−1]=(rec[x,−2]+3×rec[x,−1]+2)>>2,(x=0˜H+W−1)  [Equation 2]

Filtering may be performed on one or more reference samples among thesamples constructed as above. The filtering may be adaptively performedbased on at least one of the intra prediction mode of the current block,the size of the current block, and the shape of the current block. Forexample, at least one of a determination of whether to apply filtering,a filter type, a filter strength, and a filter coefficient may beadaptively determined.

For example, whether to apply the filtering may be determined for eachof the plurality of reference sample lines. For example, the filteringmay be applied to the first reference sample line adjacent to thecurrent block, and may not be applied to the second reference sampleline. For example, both a filtered value and an unfiltered value may beused for the same reference sample.

For example, at least one of a 3-tap filter, a 5-tap filter, a 7-tapfilter, and an N-tap filter may be selectively applied according to atleast one of the intra prediction mode of the current block, the size ofthe current block, and the shape of the current block. In this case, Mis an integer equal to or greater than 3.

For example, filters having different shapes may be selectively usedaccording to at least one of the intra prediction mode, the size, andthe shape of the current block. FIG. 9 illustrates various filtershapes.

The shape of the current block may be determined by comparing the width(horizontal dimension) of the current block with the height (verticaldimension) of the current block. For example, at least one of a decisionof whether to apply a filter, a filter type, a filter strength, and afilter coefficient may be adaptively determined according to whether thecurrent block is a horizontally oblong block or a vertically oblongblock. Alternatively, at least one of a decision of whether to applyfiltering, a filter type, a filter strength, and a filter coefficientmay be adaptively determined according to whether the current block is arectangular block or a square block.

Intra prediction for the current block may be performed based on thederived intra prediction mode and the constructed reference sample.

For example, non-directional intra prediction may be performed for thecurrent block. The mode of the non-directional intra prediction may beat least one of a DC mode, a planar mode and an LM mode.

For the DC mode, prediction may be performed using the average value ofone or more reference samples among the constructed reference samples.In this case, filtering may be applied to one or more prediction samples(also referred to as predicted samples) located at the boundary of thecurrent block. The DC prediction may be adaptively performed based on atleast one of the size of the current block and the shape of the currentblock. Further, the range of the reference samples used in the DC modecan be determined based on at least one of the size and the shape of thecurrent block.

FIG. 10 is a diagram for describing intra prediction according to theshapes of the current block.

For example, when the current block is a square block, as illustrated in(a) of FIG. 10 , DC prediction may be performed by using the averagevalue of the reference sample located above the current block and thereference sample located to the left of the current block.

For example, when the current block is a non-square block, neighboringsamples adjacent to the left end and the upper end of the current blockmay be selectively used. When the current block is a rectangular block,as illustrated in (b) of FIG. 10 , the prediction may be performed usingthe average value of the reference samples adjacent to a longer sideamong the left side and the upper side of the current block.

For example, when the size of the current block corresponds to apredetermined size or falls within a predetermined range, apredetermined number of reference samples, among the reference sampleslocated above or to the left of the current block, are selected, and theprediction is performed using the average value of the selectedreference samples. The predetermined size may be a fixed size of N×M,which is preset in the encoder/decoder. In this case, N and M areintegers greater than 0, and N and M may be the same or different fromeach other. The predetermined range may mean a threshold value forselecting the reference samples for prediction of the current block. Thethreshold value may be set with at least one of a minimum value and amaximum value. The minimum value and/or the maximum value may be a fixedvalue or fixed values preset in the encoder/decoder, or a variable valueor variable values that is/are encoded and then signaled by the encoder.

For example, one or more average values may be used to perform theprediction. When the current block is a square block or a non-squareblock, at least one of a first average value or a second average valuemay be used, in which the first average value is the average of thereference samples located above the current block and the second averagevalue is the average of the reference samples located to the left of thecurrent block. The DC prediction value of the current block may be thefirst average value or the second average value. Alternatively, the DCprediction value of the current block may be a weighted sum obtained byweighting the first average value and the second average value. Forexample, the weights for the first and second average values may be thesame (that is, 1:1).

According to the above method, a shift operation can be used tocalculate all of the DC values. For example, the method can be used evenfor the case where a sample length, which represents the width, theheight, or the sum of the width and height of the current block, is notthe power of two. The method may be applied to both luma DC predictionand chroma DC prediction. Alternatively, the method may be appliedeither to luma DC prediction or to chroma DC prediction.

For example, when the current block is a non-square block, theprediction may be performed based on either the width or the height ofthe current block. For example, a predicted value may be obtained bydividing the sum of the values of the upper reference sample and theleft reference sample by the length of a longer side (namely, the widthor the height) of the current block. In this case, the divisionoperation using the value corresponding to the longer one among thewidth and the height may be performed by a shift operation.

For example, the DC prediction may be performed using a plurality ofreference sample lines. For example, the prediction may be performedusing two reference sample lines, as illustrated in (c) of FIG. 10 .

For example, the average value of the reference samples included in thetwo reference sample lines may be determined as the DC prediction valueof the current block.

Alternatively, different weights may be applied to the reference samplesof the first adjacent line and the reference samples of the secondadjacent line of the current block. For example, a weighted average ofeach sample in the first reference sample line and each sample in thesecond reference sample line is calculated by applying the weights 3:1to each sample in the first reference sample line and each sample in thesecond reference sample line (that is, (3×the first line referencesample+the second line reference sample+2)>>2), and the average of theweighted averages may be determined as the DC prediction value of thecurrent block. Alternatively, the resultant value of ((3×the first linereference sample−the second line reference sample)>>1) may be obtained,and the average of these values may be determined as the DC predictionvalue of the current block. The weights are not limited to the aboveexample, and any weights may be used. In this case, the closer to thecurrent block the reference sample line is, the larger the weight thatis applied to the reference sample line. The number of reference samplelines that can be used is not limited to two, and three or morereference sample lines may be used for prediction.

The prediction may be performed using one or more average valuesgenerated using one or more reference samples. For example, at least oneof the average value of the reference samples in the first referencesample line located above the current block, the average value of thereference samples in the second reference sample line located above thecurrent block, the average value of the reference samples in the firstreference sample line located to the left of the current block, and theaverage value of the reference samples in the second reference sampleline located to the left of the current block may be used to perform theDC prediction.

Alternatively, the difference value between the reference sample in thefirst reference sample line and the reference sample in the secondreference sample line may be used for the DC prediction. For example,the result value of (each reference sample in the first reference sampleline+(each reference sample in the first reference sample line−eachreference sample in the second reference sample line)>>1) is calculated,and the average value of these difference values may be determined asthe DC prediction value of the current block.

For the planar mode, prediction may be performed with a weighted sum asa function of the distance from at least one reference sample to anintra prediction target sample located in the current block.

Filtering may be performed on reference samples of the current block orprediction samples (that is, predicted samples) of the current block.For example, after filtering is applied to reference samples, planarprediction may be performed, and then filtering may be performed on oneor more prediction samples. Among the prediction samples, filtering maybe performed on samples in one, two, or N sample lines located at thetop boundary or the left boundary of the current block.

To perform the planar prediction, a weighted sum of one or morereference samples may be used. For example, five reference samples maybe used, as illustrated in (d) of FIG. 10 . For example, to generate aprediction sample for a target position [x, y], the reference samplesr[−1, −1], r[x, −1], r[−1, y], r[W, −1], and r[−1, H] may be used. Inthis case, W and H are the width and the height of the current block,respectively. For example, prediction samples pred[x, y] can begenerated using Equation 3. In Equation 3, a, b, c, d, and e representweights. N may be log₂(a+b+c+d+e).

pred[x,y]=(a×r[−1,−1]+b×r[x,−1]+c×r[−1,y]+d×r[W,−1]+e×r[−1,H])>>N  [Equation3]

As another example, the planar prediction may be performed using aplurality of reference sample lines. For example, the planar predictionmay be performed using a weighted sum of two reference sample lines. Asanother example, the planar prediction may be performed using a weightedsum of reference samples in the two reference sample lines. In thiscase, the reference samples selected from the second reference sampleline may be samples adjacent to the reference samples selected from thefirst reference sample line. That is, when the reference sample locatedat the position (−1, −1) is selected, the reference sample located atthe position (−2, −2) may be selected. The planar prediction may beperformed by calculating a weighted sum of the selected referencesamples, and in this case the same weights as those used for the DCprediction may be used.

A directional prediction mode refers to at least one of a horizontalmode, a vertical mode, and an angular mode having a predetermined angle.

In the horizontal mode or the vertical mode, prediction is performedusing one or more reference samples arranged in a linear direction,i.e., in the horizontal direction or the vertical direction. A pluralityof reference sample lines may be used. For example, when two referencesample lines are used, prediction may be performed using two referencesamples arranged in a horizontal line or a vertical line. Similarly,when N reference sample lines are used, N reference samples in ahorizontal line or a vertical line may be used.

For the vertical mode, the statistics of a first reference sample (e.g.,r[x, −1]) on a first reference sample line and a second reference sample(e.g., r[x, −2]) on a second reference sample line may be used toperform the directional prediction.

For example, the predicted value of the vertical mode can be determinedby calculating the result value of (3×r[x, −1]+r[x, −2]+2)>>2.Alternatively, the predicted value of the vertical mode can bedetermined by calculating the result value of (3×r[x, −1]−r[x,−2]+1)>>1. In yet another alternative, the predicted value of thevertical mode can be determined by calculating the value of (r[x,−1]+r[x, −2]+1)>>1.

For example, the change between each of the sample values on thevertical line may be considered. For example, the predicted value of thevertical mode can be determined by calculating the result value of (r[x,−1]+(r[x, −1]−r[x, −2])>>1). In this case, N may be an integer equal toor greater than 1. As N, a fixed value may be used. Alternatively, N mayincrease with an increase in the y coordinate of a prediction targetsample. For example, N=y+1.

Even for the horizontal mode, one or more methods used for the verticalmode can be used.

For an angular mode of a certain angle, prediction may be performedusing one or more reference samples arranged in an oblique directionfrom an intra prediction target sample of the current block, or one ormore samples neighboring the reference samples located in the obliquedirection. In this case, a total of N reference samples may be used,wherein N may be 2, 3, 4, 5, or 6. It is also possible to performprediction by applying at least one of an N-tap filter to the Nreference samples. Examples of the N-tap filter include a 2-tap filter,a 3-tap filter, a 4-tap filter, a 5-tap filter, and a 6-tap filter. Atthis time, at least one of the reference samples may be located abovethe current block and the rest may be located to the left of the currentblock. The reference samples located above the current block (or thereference samples located to the left of the current block) may belocated in the same line or in different lines.

According to another embodiment, intra prediction may be performed basedon position information. In this case, the position information may beencoded/decoded, and a reconstructed sample block located at theposition described above may be derived as an intra predicted block ofthe current block. Alternatively, a block similar to the current blockmay be searched for by the decoder, and the found block may be derivedas the intra predicted block of the current block.

According to a further embodiment, inter color component intraprediction is performed. For example, it is possible to intra-predictchroma components from the corresponding reconstructed luma component ofthe current block. Alternatively, it is possible to intra-predict onechroma component Cr from the corresponding reconstructed chromacomponent Cb of the current block.

In the various embodiments described above, reference samples that arenot filtered may be used in the reference sample construction processfor intra prediction. That is, directional prediction or non-directionalprediction may be performed by using non-filtered reference samples.Since filtering is not performed in the reference sample constructionprocess, the complexity of the encoder/decoder can be reduced and thehardware configuration can be simplified.

In intra prediction, one or more reference samples may be used forinterpolation prediction. In intra prediction, at least one of theparameters among the number of reference sample lines, the number ofinterpolation filter taps, an interpolation filter coefficient,information on application/non-application of a filter, a weightedaverage calculation method, and weights may vary depending on at leastone of the intra prediction mode of the current block, the size of thecurrent block, and the shape of the current block.

For example, one or more reference sample lines may be used, and thenumber of used reference sample lines may vary depending on one or morecoding parameters. A plurality of reference sample lines may be used.FIG. 11 is a diagram illustrating an embodiment in which two referencesample lines are used.

The number of reference sample lines used for prediction variesdepending on the intra prediction mode of the current block or thedirectionality of the intra prediction mode. For example, when the intraprediction mode of the current block is a non-directional mode such as aDC mode or a Planar mode, one reference sample line may be used. Whenthe intra prediction mode of the current block is a directional mode,two reference sample lines may be used. For example, the number ofreference sample lines may vary depending on whether the directionalmode meets a predetermined condition or falls within a predeterminedrange. That is, when the directional mode is an even-numbered mode, tworeference sample lines may be used. When the directional mode is anodd-numbered mode, one reference sample line may be used. For example,one reference sample line may be used for the horizontal mode or thevertical mode. For example, when the intra prediction mode falls withina predetermined range, a plurality of reference sample lines may beused. Otherwise, a single reference sample line may be used.

The number of reference sample lines may vary depending on the sizeand/or shape of the current block. For example, one reference sampleline may be used when the current block is smaller than a predeterminedsize, and two reference sample lines may be used when the current blockis larger than the predetermined size. That is, one reference sampleline may be used when the current block is smaller than or equal to asize of 16×16 (that is, 256 samples), and two reference sample lines maybe used when the current block is larger than the size of 16×16.Conversely, two reference sample lines may be used for a smaller blockand one reference sample line may be used for a larger block. Forexample, two reference sample lines may be used when the current blockis a square block, and one reference sample line may be used when thecurrent block is a non-square block. The number of reference samplelines may vary depending on the location of the reference sample, thatis, depending on whether the reference sample is located above thecurrent block or to the left of the current block.

Alternatively, the number of reference sample lines may vary dependingon the width (horizontal dimension) or the height (vertical dimension)of the current block. For example, when the width or the height of thecurrent block is greater than a predetermined value, a plurality ofreference sample lines may be used. In contrast, when the width orheight of the current block is equal to or less than the predeterminedvalue, one reference sample line may be used. For example, only onereference sample line may be used when the current block is a 4×N-sizeor N×4-size block, and two or more reference sample lines may be usedotherwise. The same weight may be applied to two or more referencesample lines. For example, when the statistics of the width and heightof the current block are within a predetermined range, a plurality ofreference sample lines may be used. Otherwise, one reference sample linemay be used.

The number of reference sample lines may vary depending on the intraprediction mode of the current block and the length (horizontal orvertical dimension, that is, width or height) of the current block. Forexample, when the intra prediction mode is the vertical direction mode,the number of reference sample lines may vary depending on the width(horizontal dimension) of the current block. For example, when the intraprediction mode of the current block is the horizontal direction mode,the number of reference sample lines may vary depending on the height(vertical dimension) of the current block. For example, when the intraprediction mode of the current block is the non-directional mode, thenumber of reference sample lines may vary depending on whether or notthe length of the horizontal dimension and/or the vertical dimension ofthe current block corresponds to a predetermined value or falls within apredetermined range.

The number of reference sample lines may vary depending on the colorcomponent of the current block. For example, a plurality of (that is,two or more) reference sample lines may be used for the luma component,and one reference sample line may be used for each of the chromacomponents.

The number of reference sample lines may be differently set when theboundary of the current block corresponds to the boundary of apredetermined unit. For example, when the top boundary of the currentblock corresponds to the boundary of at least one of a picture, a slice,a tile, a coding tree unit (CTU), and an arbitrary-size block, onereference sample line may be used for the samples in the top boundary ofthe current block. Similarly, when the left boundary of the currentblock corresponds to the boundary of at least one of a picture, a slice,a tile, a coding tree unit (CTU), and an arbitrary-size block, onereference sample line may be used for the samples in the left boundaryof the current block. The arbitrary block size may be signaled orpredefined in the encoder/decoder.

When a plurality of reference sample lines are used, which referencesample lines are used may be set differently based on the codingparameter (for example, intra prediction mode). For example, a firstreference sample line may be used when the intra prediction mode of thecurrent block is an even-numbered mode, and a second reference sampleline may be used when the intra prediction mode of the current block isan odd-numbered mode.

An interpolation filter may be used when performing directionalprediction on the current block. The interpolation filter may be afilter having at least one of 2 taps, 4 taps, 6 taps, and N taps (Nbeing a positive integer). Each of the interpolation filter taps has oneor more filter coefficients.

For example, a 6-tap filter may be applied to samples S00 to S05 of FIG.11 according to Equation 4, and the filter coefficients may range from ato f.

S_F=(a×S00+b×S01+c×S02+d×S03+e×S04+f×S05+2^(g-1))>>g  [Equation 4]

The sum of the filter coefficients may be at least one of 32, 64, 128,256, 512, 1024, and N, and each filter coefficient may be an integervalue. The sum of the filter coefficients may be equal to the g-th powerof 2. For example, when the sum of the filter coefficients is 1024, gmay be 10.

The interpolation filter tap or the coefficient may vary depending on atleast one of the size of the current block, the shape of the currentblock, the position of a prediction target sample, and the intraprediction mode of the current block.

The interpolation filter tap or the coefficient may vary depending onthe intra prediction mode. For example, a 4-tap filter may be appliedwhen the intra prediction mode is a predetermined mode, and a 2-tapfilter may be applied when the intra prediction mode is not thepredetermined mode. In addition, the filter coefficient may varydepending on the angle of the intra prediction mode. For example, for a6-tap filter, two filter types (that is, Filter1 and Filter2) may beused, as shown in Table 1. The 6-tap filter may have filter coefficients{a, b, c, d, e, f} and the filter coefficients may be stored in the formof a lookup table (LUT). In this case, index information for referringto the lookup table may be encoded/decoded.

TABLE 1 6-tab Filter1 6-tab Filter2 angle 0 {0, 256, 512, 256, 0, 0}{47, 255, 416, 256, 49, 1} angle 1 {−3, 246, 509, 267, 6, −1} {43, 247,416, 264, 53, 1} angle 2 {−5, 237, 506, 278, 11, −3} {40, 240, 414, 270,58, 2} angle 3 {−7, 228, 502, 288, 17, −4} {37, 233, 413, 277, 62, 2}angle 4 {−9, 218, 497, 299, 24, −5} {34, 226, 412, 284, 66, 2} angle 5{−10, 210, 493, 309, 29, −7} {31, 218, 410, 292, 71, 2} angle 6 {−12,200, 488, 320, 36, −8} {28, 210, 407, 299, 77, 3} angle 7 {−13, 191,482, 330, 43, −9} {26, 203, 404, 306, 82, 3} angle 8 {−14, 182, 476,340, 50, −10} {23, 195, 401, 313, 88, 4} angle 9 {−15, 173, 470, 350,57, −11} {21, 188, 398, 320, 93, 4} angle 10 {−16, 163, 463, 361, 65,−12} {19, 180, 393, 327, 100, 5} angle 11 {−16, 155, 456, 370, 72, −13}{17, 173, 389, 333, 106, 6} angle 12 {−16, 147, 449, 379, 79, −14} {16,167, 385, 339, 111, 6} angle 13 {−16, 138, 440, 388, 88, −14} (14, 159,380, 346, 118, 7} angle 14 {−17, 128, 433, 399, 96, −15} {13, 153, 375,351, 124, 8} angle 15 {−16, 121, 425, 407, 103, −16} (11, 145, 370, 358,131, 9} angle 16 {−16, 112, 416, 416, 112, −16} {10, 138, 364, 364, 138,10}

As can be seen from Table 1, reference samples close to the angle lineof the directional mode can be given a large weight. For example, afirst filter may be applied when the intra prediction mode of thecurrent block is an even-numbered mode, and a second filter may beapplied when the intra prediction mode of the current block is anodd-numbered mode.

Alternatively, the first filter may be applied when the intra predictionmode is a mode having an angle corresponding to a multiple of 45degrees, and the second filter may be applied when the intra predictionmode is a mode having one of the other angles. The first filter and thesecond filter differ in terms of at least one of the filter tap, thefilter coefficient, and the filter shape.

The interpolation filter tap or the coefficient may vary depending onthe position of a prediction target sample within the current block. Forexample, a first interpolation filter may be applied when the positionof the prediction target sample is close to the reference sample, and asecond interpolation filter may be applied when the position of theprediction target sample is far from the reference sample. A third, afourth, . . . , and an N-th interpolation filter may be appliedaccording to the position of the prediction target sample, and thenumber of interpolation filters may vary depending on the size and/orshape of the current block. The plurality of interpolation filters mayhave the same filter tap and different filter coefficients.Alternatively, the plurality of interpolation filters may have differentfilter taps and different filter coefficients.

For example, the first filter may be applied when the width or height ofthe current block has a first length, and the second filter may beapplied when the width or height of the current block has a secondlength.

For example, the first filter may be applied when the width or height ofthe current block corresponding to the reference sample region used inthe directional prediction mode is less than or equal to 8, and thesecond filter may be applied when the width or height is greater than 8.

For example, when the block size is 64 or less, the first filter may beused. Otherwise, the second filter may be used. The two types of filtersmay be selectively used based on the location of the prediction targetsample. For example, when the intra prediction mode falls within a rangeof 34 to 66, the first filter may be applied to a prediction targetsample located at an upper part of the current block, and the secondfilter may be applied to a prediction target sample located at a lowerpart of the current block. Similarly, when the intra prediction modefalls within a range of 2 to 33, the first filter may be applied to aprediction target sample located at a left part of the current block,and the second filter may be applied to a prediction target samplelocated at a right part of the current block. In the above embodiment,the first filter may be a cubic interpolation filter and the secondfilter may be a Gaussian interpolation filter.

For example, the filter coefficients can be adaptively selected andapplied according to the width or height of the current block.

The interpolation filter tap or the filter coefficient may varydepending on the color component of the current block. For example, afirst filter may be applied to a luma signal sample, and a second filtermay be applied to chroma signal samples. For example, a 4-tap filter maybe applied to a luma signal sample, and a 2-tap (bilinear) filter may beapplied to chroma signal samples.

When a plurality of reference sample lines is used, the interpolationfilter tap or the filter coefficient may vary depending on whichreference sample line is used. For example, a first filter may beapplied to a first reference sample line adjacent to the current block,and a second filter may be applied to a second reference sample line.When performing directional prediction as illustrated in FIG. 11 , afirst interpolation filter may be applied to samples S00 to S05 locatedin the first reference sample line, and a second interpolation filtermay be applied to samples S12 to S13 located in a second referencesample line.

For example, a first filter may be applied to all of the plurality ofreference sample lines. In this case, a filter coefficient applied tothe first reference sample line and a filter coefficient applied to thesecond reference sample line may be different from each other.

At this time, when the direction of a directional prediction mode passesa predetermined position between two reference samples, referencesamples selected from respective reference sample lines may differ interms of the locations thereof. For example, a 2-tap interpolationfilter is performed using samples S02 and S03 located in a firstreference sample line, and a 2-tap interpolation filter is performedusing samples S13 and S14 located in the second reference sample line.

When a plurality of reference sample lines is used, the interpolationfilter may have a two-dimensional shape. For example, a first filterhaving the same shape as the example illustrated in FIG. 9 may beapplied.

For example, reference samples which are not filtered by aninterpolation filter may be used for directional prediction. Forexample, when a reference sample corresponding to a directionalprediction mode exists at an integer position, the reference samplewhich is not filtered by an interpolation filter can be used. At leastone of the 3-tap, 5-tap, and N-tap filters may be applied to thereference sample to which the interpolation filter is not applied. Forexample, a {1, 2, 1} filter may be applied to a reference sample.Whether to apply the filter to a reference sample may be determinedbased on at least one of the intra prediction mode of the current block,the size of the current block, and the shape of the current block.

For example, when a plurality of reference sample lines is used, aninterpolation filter or a weighted average may be applied to a pluralityof values obtained by applying an interpolation filter to each of thereference sample lines. For example, as in Equation 5, a weightedaverage of values S-F1 and S_F2 may be derived as a prediction value SP, in which the value S_F1 is obtained by applying a first interpolationfilter to a first reference sample line and the value S_F2 is obtainedby applying a second interpolation filter to a second reference sampleline. Here, h and i may be weights, and h+i may be a value correspondingto the j-th power of 2. For example, h=3, i=1, and j=2. Alternatively,h, i, and j may all be 1. The first interpolation filter and the secondinterpolation filter may be of different filter types. For example, thefirst interpolation filter and the second interpolation filter maydiffer in terms of at least one of the filter tap and the filtercoefficient.

S_P=(h×S_F1+i×S_F2+2^(j-1))>>j  [Equation 5]

Alternatively, a value (for example, S_F1+(S_F1−S_F2)>>j) obtained byconsidering the amount of change between each of the angular lines, suchas the vertical prediction, may be determined as a predicted value.

In the above, it has been described that different interpolations areperformed for a first reference sample line adjacent to the currentblock and for a second reference sample line, among a plurality ofreference sample lines. However, the operation described above is notlimited to the first reference sample line and the second referencesample line among the plurality of reference sample lines. For example,the first reference sample line and the second reference sample line maybe replaced with an arbitrary first reference sample line and anarbitrary second reference sample line, respectively, of the pluralityof reference sample lines.

In applying the interpolation filters, padding may be performed whenusing samples that are positioned outside of the configured referencesample area. For example, when a directional prediction mode correspondsa direction passes between reference samples S04 and S05 in FIG. 11 ,when applying a 6-tap filter, two samples positioned on the right sideand deviated from the direction of the directional prediction mode maybe padded with the available reference sample S05, and then theinterpolation filter may be applied. In case of an angular mode, theconfigured reference sample may be re-configured based on an angularprediction mode. For example, when the angular prediction mode is a modeusing all of left side and upper side reference samples, aone-dimensional array may be configured for the left side or upper sidereference sample. Alternatively, an upper side reference sample may beconfigured by shifting a left side reference sample, or an upper sidereference sample may be configured by using a weighted sum of at leastone left side reference sample.

Angular intra predictions or interpolation filterings different fromeach other may be performed on a predetermined sample group unit of acurrent block. The predetermined sample group unit may be a block, asub-block, a line or a singular sample.

According to another embodiment of the present invention, an inter colorcomponent intra prediction may be performed. The inter color componentintra prediction includes a color component block restructuring step, aprediction parameter deriving step, and/or an inter color componentprediction execution step. The term ‘color component’ may refer to atleast any one of a luma signal, a chroma signal, Red, Green, Blue, Y,Cb, and Cr. A prediction of a first color component can be performed byusing at least any one of a second color component, a third colorcomponent, and a fourth color component. The signals of the colorcomponents used for the prediction may include at least any one of anoriginal signal, a reconstructed signal, a residual signal, and aprediction signal.

When performing an intra prediction for a second color component targetblock, a sample of a first color component block corresponding blockthat corresponds to the second color component target block, a sample ofa neighbor block of the first color component corresponding block, orboth of the samples may be used. For example, when performing an intraprediction for a chroma component block Cb or Cr, a reconstructed lumacomponent block Y corresponding to the chroma component block Cb or Crmay be used.

When predicting the chroma components on the basis of the lumacomponent, the prediction may be performed according to Equation 6.

Pred_(C)(i,j)=α·rec_(L)′(i,j)+β  [Equation 6]

In Equation 6, Pred_(C)(i, j) represents a predicted chroma sample ofthe current block, and rec_(L)(i, j) represents a reconstructed lumasample of the current block. At this time, rec_(L)′(i, j) may be adown-sampled reconstructed luma sample. Parameters α and β may bederived by minimizing a regression error between the reconstructedneighboring luma sample and the reconstructed neighboring chroma samplearound the current block.

There are two modes for predicting the chroma components using the lumacomponent. The two modes may include a single-model mode and amultiple-model mode. The single-model mode may use one linear model whenpredicting the chroma components from the luma components for thecurrent block. The multiple-model mode may use two linear models.

In the multiple-model mode, the samples adjacent to the current block(that is, adjacent luma samples and adjacent chroma samples) may beclassified into two groups. That is, the parameters α and β for each ofthe two groups may be derived. Further, the luma samples of the currentblock may be classified according to the rules used for classificationof the luma samples adjacent to the current block.

For example, a threshold value for classifying the adjacent samples intotwo groups may be calculated. The threshold value may be calculatedusing an average value of the reconstructed adjacent luma samples.However, the calculation of the threshold value is not limited thereto.At least one of various statistical values recognized in the presentspecification may be used instead of the average value. When the valuesof the adjacent samples are larger than the threshold value, theadjacent samples may be classified into a first group. Otherwise, theadjacent samples may be classified into a second group.

Although it is described that the multiple-model mode uses two linearmodels in the embodiment described above, the present invention is notlimited thereto, and may cover other cases in which two or more linearmodels are used. When N linear models are used, samples may beclassified into N groups. To do so, N−1 threshold values may becalculated.

As described above, when predicting a chroma component from a lumacomponent, a linear model can be used. In this case, the linear modelmay include a simple linear model (hereinafter referred to as “LM1”), acomplex linear model (hereinafter referred to as “LM2”), and a complexfilter linear model (hereinafter, referred to as “LM3”). Parameters ofthe models described above may be derived by minimizing regression errorbetween the reconstructed luma samples around the current block and thecorresponding reconstructed chroma samples around the current block.

FIG. 12 is a diagram for describing “neighboring samples of a currentblock” (hereinafter referred to as “adjacent data set”) used to derivethe parameters of the models.

The adjacent data set for deriving the parameters of the LM1 may becomposed of a pair of samples comprising a luma sample and a chromasample in each of a line area B and a line area C illustrated in FIG. 12. The adjacent data set for deriving the parameters of the LM2 and LM3may be composed of a pair of samples comprising a luma sample and chromasample in each of a line area B, a line area C, a line area E, and aline area F illustrated in FIG. 12 .

However, the adjacent data set is not limited to the examples describedabove. For example, to cover various linear relationships between lumaand chroma samples in the current block, N adjacent data sets may beused for each mode. For example, N may be an integer of 2 or more, andspecifically 3.

The parameters of the linear model may be calculated using both an uppertemplate and a left template. Alternatively, there are two LM modes (anLM A mode and an LM L mode), and the upper template and the lefttemplate may be used in the LM A mode and the LM L mode, respectively.That is, in the LM A mode, the linear model parameters may be obtainedusing only the upper template. When the position of the upper leftcorner sample of the current block is (0, 0), the upper template may beextended to a range from (0, −n) to (W+H−1, −n). In this case, n is aninteger equal to or greater than 1. That is, in the LM L mode, thelinear model parameters may be obtained using only the left template.The left template may be extended to a range from (−n, 0) to (−n,H+W−1). In this case, n is an integer equal to or greater than 1.

A power of two numbers of samples can be used to derive the parametersof the linear model. When the current chroma block is a non-squareblock, the samples used to derive the parameters of the linear model maybe determined based on the number of samples on a shorter side, amongthe horizontal side and the vertical side of the current block.According to one embodiment, when the size of the current block is n×m(where n>m), m samples of the n adjacent samples adjacent to the topboundary of the current block may be selected, for example, byperforming sub-sampling uniformly. In this case, the number of samplesused to derive the parameters of the linear model may be 2m. As anotherexample, when the size of the current block is n×m (where n>m), msamples of the n adjacent samples adjacent to the top boundary of thecurrent block may not be used. For example, of the n samples, m samplesthat are farthest from the shorter one of the horizontal side and thevertical side of the current block may not be used. In this case, thenumber of samples used to derive the parameters of the linear model maybe n (n-m samples adjacent to the top boundary of the current block+msamples adjacent to the left boundary of the current block).

Alternatively, when performing an intra prediction for a chromacomponent block Cr, a chroma component block Cb may be used.Alternatively, when performing an intra prediction for a fourth colorcomponent block, at least one of a first color component block, a secondcolor component block, and a third color component, all of whichcorrespond to the fourth color component block, may be used.

Whether or not to perform an inter color component intra prediction maybe determined based on at least any one of the size and the shape of acurrent target block. For example, when the size of the target block isequal to that of a coding tree unit (CTU), larger than a predeterminedsize, or within a predetermined size range, the inter color componentintra prediction for the target block can be performed. Alternatively,when the shape of the target block is a predetermined shape, the intercolor component intra prediction for the target block can be performed.The predetermined shape may be a square shape. In this case, when thetarget block has an oblong shape, the inter color component intraprediction for the target block may not be performed. Meanwhile, whenthe predetermined shape is an oblong shape, the embodiment describedabove inversely operates.

Alternatively, whether or not to perform an inter color component intraprediction for a prediction target block may be determined based on acoding parameter of at least any one block selected from among acorresponding block corresponding to the prediction target block andneighbor blocks of the corresponding block. For example, when thecorresponding block has been predicted through an intra predictionmethod in a constrained intra prediction (CIP) environment, an intercolor component intra prediction for the prediction target block may notbe performed. Alternatively, when the intra prediction mode of thecorresponding block is a predetermined mode, an inter color componentintra prediction for the prediction target block can be performed.Further alternatively, whether or not to perform an inter colorcomponent intra prediction may be determined on the basis of at leastany one of CBF information of the corresponding block and CBFinformation of the neighbor blocks thereof. The coding parameter is notlimited to a prediction mode of a block but various parameters that canbe used for encoding/decoding may be used.

The color component block restructuring step will be described below.

When predicting a second color component block by using a first colorcomponent block, the first color component block may be restructured.For example, when an image has an YCbCr color space and when a samplingratio of color components is one of 4:4:4, 4:2:2, and 4:2:0, the blocksizes of color components may differ from each other. Therefore, whenpredicting a second color component block using a first color componentblock having a different size from the second color component block, thefirst color component block may be restructured such that the blocksizes of the first color component and the second color component areequalized. The restructured block may include at least any one of asample in the first color component block that is a corresponding blockand a sample in a neighbor block of the first color component block.FIG. 13 is an exemplary diagram illustrating a process of restructuringa color component block.

In (a) of FIG. 13 , p1[x, y] represents a sample at a position (x, y) inthe first color component block. In (b) of FIG. 13 , p1′[x, y]represents a sample at a position (x, y) in the restructured block thatis produced by restructuring the first color component block.

When the first color component block has a larger size than the secondcolor component block, the first color component block is down-sampledto have a size equal to that of the second color component block. Thedown-sampling may be performed by applying an N-tap filter to one ormore samples (N is an integer equal to or larger than 1). For thedown-sampling, at least any one equation of Equation 7 to Equation 11may be used. In the case in which any one down-sampling method amongvarious down-sampling methods is selectively used, an encoder may selectone down-sampling method as a predetermined down-sampling method. Forexample, the encoder may select a down-sampling method having optimaleffects. The selected down-sampling method is encoded and signaled to adecoder. The signaled information may be index information indicatingthe down-sampling method.

p1′[x,y]=(p1[2x,2y]+p1[2x,2y+1]+1)>>1  [Equation 7]

p1′[x,y]=(p1[2x+1,2y]+p1[2x+1,2y+1]+1)>>1  [Equation 8]

p1′[x,y]=(p1[2x−1,2y]+2×p1[2x,2y]+p1[2x+1,2y]+2)>>2  [Equation 9]

p1′[x,y]=(p1[2x−1,2y+1]+2*p1[2x,2y+1]+p1[2x+1,2y+1]+2)>>2  [Equation 10]

p1′[x,y]=(p1[2x−1,2y]+2*p1[2x,2y]+p1[2x+1,2y]+p1[2x−1,2y+1]+2*p1[2x,2y+1]+p1[2x+1,2y+1]+4)>>3  [Equation11]

The down-sampling method performed with respect to two or more samplesis not limited to any one of the examples of Equation 7 to Equation 11.For example, two or more samples used to calculate a down-sampled valuep1′[x, y] may be selected from a sample group consisting of a samplep1[2x, 2y] and neighbor samples thereof. The neighbor samples may beones selected among p1[2x−1, 2y−1], p1[2x−1, 2y], p1[2x−1, 2y+1], p1[2x,2y−1], p1[2x, 2y+1], p1[2x+1, 2y−1], p1[2x+1, 2y], and p1[2x+1, 2y+1].The down-sampling can be performed by calculating the average or theweighted average of two or more samples.

Alternatively, the down-sampling may be performed in a manner ofselecting a specific sample among one or more samples. In this case, atleast any one of the following equations, Equation 12 to Equation 15,may be used for the down-sampling.

p1′[x,y]=p1[2x,2y]  [Equation 12]

p1′[x,y]=p1[2x,2y+1]  [Equation 13]

p1′[x,y]=p1[2x+1,2y]  [Equation 14]

p1′[x,y]=p1[2x+1,2y+1]  [Equation 15]

When the first color component block has a smaller size than the secondcolor component block, the first color component block is up-sampled tobe restructured such that the sizes of the first color component blockand the second color component block are equalized. In this case, theup-sampling is performed according to Equation 16.

p1′[2×,2y]=p1[x,y],

p1′[2x+1,2y]=(p1[x,y]+p1[x+1,y]+1)>>1,

p1′[2x,2y+1]=(p1[x,y]+p1[x,y+1]+1)>>1,

p1′[2x+1,2y+1]=(p1[x+1,y]+p1[x,y+1]+1)>>1  [Equation 16]

In the restructuring process, a filter may be applied to one or moresamples. For example, the filter may be applied to one or more samplesincluded in at least any one of the first color component block (i.e.corresponding block), neighbor blocks of the corresponding block, thesecond color component block (i.e. target block), and neighbor blocks ofthe target block.

In the reference sample restructuring step described above, an indicatorcorresponding to a predetermined reference sample line among a pluralityof reference sample lines may be signaled. In this case, in therestructuring process, the restructuring is performed using thepredetermined reference sample line corresponding to the signaledindicator.

In the restructuring process, when a boundary of the second colorcomponent block (target block) or a boundary of the first colorcomponent block (corresponding block) is a boundary of a predeterminedregion, the reference samples used for the restructuring may bedifferently selected. In this case, the number of reference sample linesat the upper side may differ from the number of reference sample linesat the left side. The predetermined region may be at least any one of apicture, a slice, a tile, a CTU, and a CU.

For example, when the upper boundary of the first color componentcorresponding block is the boundary of the predetermined region, thereference samples at the upper side may not be used for therestructuring but only the reference samples at the left side may beused for the restructuring. When the left boundary of the first colorcomponent corresponding block is the boundary of the predeterminedregion, the reference samples at the left side may not be used for therestructuring but only the reference samples at the upper side may beused for the restructuring. Alternatively, both of N reference samplelines at the upper side and M reference sample lines at the left sidemay be used for the restructuring, in which N may be smaller than M. Forexample, when the upper boundary corresponds to the boundary of thepredetermined region, N may be 1. Meanwhile, when the left boundarycorresponds to the boundary of the predetermined region, M may be 1.

Alternatively, the restructuring may be performed by using N referencesample lines at the upper side and M reference left sample lines at theleft side of the first color component corresponding block, regardlessof whether the boundary of the predetermined region is the upperboundary or the left boundary of the first color component block.

FIG. 14 is a diagram illustrating an embodiment performing restructuringby using a plurality of upper-side reference sample lines and/or aplurality of left-side reference sample lines.

As illustrated in (a) of FIG. 14 , the restructuring may be performedusing four upper-side reference sample lines and four left-sidereference sample lines.

For example, when the upper boundary or the left boundary of the firstcolor component corresponding block is the boundary of the predeterminedregion, the number of the upper-side reference sample lines and thenumber of the left-side reference sample lines used for therestructuring may differ from each other. For example, as illustrated in(b) to (d) of FIGS. 14 , any of the following combinations may be usedfor the restructuring: two upper-side reference sample lines and fourleft-side reference sample lines; one upper-side reference sample lineand three left-side reference sample lines; and one upper-side referencesample line and two left-side reference sample lines.

The number of reference sample lines used for the restructuring is notlimited to the above combinations. That is, N upper-side referencesamples lines and M left-side reference sample lines may be used inwhich N and M are equal to or different from each other. When both ofthe upper boundary and the left boundary of the corresponding blockcorrespond to the boundary of the predetermined region, N and M may beequal to each other. That is, N and M may be both 1. Alternatively, Nmay be set smaller than M under the same condition. This is because moreresources (memory) are required for the upper-side reference samplelines than for the left-side reference sample lines.

Alternatively, as illustrated in (e) of FIG. 14 , one or more referencesamples within a region having a vertical length and a horizontal lengthnot larger than those of the first color component corresponding blockmay be used for the restructuring.

When performing the restructuring process, the reference samples of thefirst color component corresponding block may be differently setdepending on any one of the block size, the block shape, and the codingparameter of at least any one block selected among the first colorcomponent corresponding block, neighbor blocks thereof, the second colorcomponent target block, and neighbor blocks thereof.

For example, among samples in the first color component correspondingblock and the neighbor blocks thereof, samples in blocks whose encodingmode is an inter frame encoding mode are not used but only samples inblocks whose encoding mode is an intra encoding mode are used for therestructuring.

FIG. 15 is an exemplary diagram illustrating reference samples used forthe restructuring in accordance with an intra prediction mode or acoding parameter of a corresponding block. The restructuring of thereference samples of the first color component block may be differentlyperformed in accordance with the intra prediction modes of the firstcolor component corresponding block. For example, when the intraprediction mode of the corresponding block is a non-angular mode, suchas a DC mode and a planar mode, or an angular mode in which both of theupper-side reference samples and the left-side reference samples areused, as illustrated in (a) of FIG. 15 , at least one of the upper-sidereference samples and the left-side reference samples is used for therestructuring. Alternatively, when the intra prediction mode of thecorresponding block is an angular mode in which both of the upper-sidereference samples and the upper-right-side reference samples are used,as illustrated in (b) of FIG. 15 , at least one of the upper-sidereference samples and the upper right-side reference samples is used forthe restructuring. Alternatively, when the intra prediction mode of thecorresponding block is an angular mode in which both of the left-sidereference samples and the lower left-side reference samples are used, asillustrated in (c) of FIG. 15 , at least one of the left-side referencesamples and the lower left-side reference samples is used for therestructuring.

Alternatively, the reference samples used to restructure the first colorcomponent corresponding block are differently selected in accordancewith the quantization parameter of at least any one of the first colorcomponent corresponding block and the neighbor blocks thereof. Forexample, as illustrated in (d) of FIG. 15 , reference samples in anupper block that is disposed at the upper side of the correspondingblock and whose neighbor blocks have a relatively small quantizationparameter value QP are used for the restructuring of the correspondingblock.

Alternatively, when the second color component target block has anoblong shape, reference samples disposed around a first color componentcorresponding block having a square shape are used for therestructuring.

Alternatively, when the second color component target block ispartitioned into two sub-blocks (for example, two 16×8-size sub-blocks)and when the first color component corresponding block is a 32×16-sizeblock, reference samples disposed around a 32×32-size block are used forthe restructuring of the corresponding block. In this case, as referencesamples of the first color component block corresponding to a second16×8-size sub-block disposed at a lower side among the partitioned twosub-blocks of the second color component target block, reference samplesaround a restructured 32×32-size block may be shared.

Hereinbelow, the prediction parameter deriving step will be described.

A prediction parameter can be derived using at least any one ofreference samples of the restructured first color componentcorresponding block and reference samples of the second color componentprediction target block. Hereinafter, the terms ‘first color component’and ‘first color component block’ may respectively refer to arestructured first color component and a restructured first colorcomponent block.

FIG. 16 is a diagram illustrating an exemplary restructured first colorcomponent corresponding block when a second color component predictiontarget block is a 4×4 block. In this case, the number of referencesample lines may be N.

The prediction parameter may be derived using reference samples disposedat the upper side and the left side of the restructured first colorcomponent corresponding block or of the second color componentprediction target block as illustrated in (a) of the FIG. 16 .

For example, the prediction parameter can be derived by adaptively usingthe reference samples of the restructured first color component, on thebasis of the intra prediction mode of the first color componentcorresponding block. In this case, the reference samples of the secondcolor component can be adaptively used on the basis of the intraprediction mode of the first color component corresponding block.

When the intra prediction mode of the first color componentcorresponding block is a non-angular mode such as a DC mode or a planarmode, or an angular mode in which both of upper-side reference samplesand left-side reference samples are used, reference samples at the upperside and the left side of the first color component corresponding blockcan be used as illustrated in (a) of FIG. 16 .

When the intra prediction mode of the first color componentcorresponding block is a non-angular mode in which upper-side referencesamples are used, reference samples at the upper side of the first colorcomponent corresponding block may be used as illustrated in (b) or (c)of FIG. 16 .

When the intra prediction mode of the first color componentcorresponding block is an angular mode in which left side referencesamples are used, reference samples at the left side of the first colorcomponent corresponding block may be used as illustrated in (d) or (e)of FIG. 16 .

Alternatively, when the intra prediction mode of the first colorcomponent corresponding block is an angular mode, reference samples usedin each prediction mode can be used as reference samples of the firstcolor component. For example, when the intra prediction mode is avertical mode, reference samples illustrated in (b) of FIG. 16 may beused. When the intra prediction mode is a horizontal mode, referencesamples illustrated in (d) of FIG. 16 may be used. When the intraprediction mode is an up-right diagonal mode, reference samplesillustrated in (c) of FIG. 16 may be used. When the intra predictionmode is a down-left diagonal mode, reference samples illustrated in (e)of FIG. 16 may be used. When the intra prediction mode is a mode betweenthe vertical mode and the up-right diagonal mode, reference samplesillustrated in (f) of FIG. 16 may be used. When the intra predictionmode is an angular mode of a 45° diagonal direction, upper rightreference samples, lower left reference samples, or both are used asillustrated in (g) of FIG. 16 . Reference samples that are differentlyselected for each intra prediction mode are stored in a format of alook-up table so as to be conveniently used.

The prediction parameter may be derived by adaptively using thereference samples of the first color component or the second colorcomponent in accordance with the size and/or the shape of the firstcolor component block and/or the second color component block.

For example, when the second color component target block has a 64×64size, 32, 16, or 8 reference samples among reference samples at theupper side or the left side of the first color component block or thesecond color component block may be used. As described above, when thesize of the second color component target block is a predetermined size,the reference samples of the first or second color component block maybe adaptively used. The predetermined size is not limited to the 64×64size, but it may be a size signaled through a bitstream or a sizederived on the basis of the coding parameter of a current block or aneighbor block thereof.

Alternatively, when the second color component target block has anoblong shape, reference samples adjacent to a longer side, which is avertical side or a horizontal side, of the second color component targetblock may be used. For example, when the target block has a block sizeof 32×8, reference samples at the upper side of the first colorcomponent or the second color component block may be used.

Alternatively, when the second color component target block has anoblong shape, reference samples around a square block can be used. Forexample, when the target block is a 32×8 block, reference samples arounda 32×32 block can be used.

The prediction parameter can be derived using reference samples aroundthe restructured first color component block and reference samplesaround the second color component block. The prediction parameter can bederived on the basis of any one of the factors including a correlation,a change, an average value, and a distribution of color components. Inthis case, any one of the methods of Least Squares (LS), Least MeanSquares (LMS), etc. may be used.

When deriving the prediction parameters through the LMS method, theprediction parameters may be a and b, α and β, or both. Predictionparameters that can minimize an error between the reference samples ofthe first color component and the reference samples of the second colorcomponent can be derived by Equation 17.

$\begin{matrix}{{E( {a,b} )} = {\sum\limits_{n = 0}^{N - 1}( {{p2_{n}} - ( {{{a \cdot p}1_{n}^{\prime}} + b} )} )^{2}}} & \lbrack {{Equation}17} \rbrack\end{matrix}$

In Equation 17, p2_(n) represents a reference sample of the second colorcomponent, and p1′_(n) represents a reference sample of the restructuredfirst color component. N is the number of used reference samplesarranged in a vertical direction or a horizontal direction, and a and brepresent prediction parameters.

In this case, a correlation between the reference samples can becalculated by Equation 18.

$\begin{matrix}{{k = {{Max}( {0,{{BitDepth} + {\log 2(N)} - 15}} )}}{L = {( {{\sum\limits_{y = 0}^{N - 1}{p{1^{\prime}\lbrack {{- 1},y} \rbrack}}} + {\sum\limits_{x = 0}^{N - 1}{p{1^{\prime}\lbrack {x,{- 1}} \rbrack}}}} )\operatorname{>>}k}}{C = {( {{\sum\limits_{y = 0}^{N - 1}{p{2^{\prime}\lbrack {{- 1},y} \rbrack}}} + {\sum\limits_{x = 0}^{N - 1}{p{2^{\prime}\lbrack {x,{- 1}} \rbrack}}}} )\operatorname{>>}k}}{{LL} = {( {{\sum\limits_{y = 0}^{N - 1}{p{1^{\prime}\lbrack {{- 1},y} \rbrack}2}} + {\sum\limits_{x = 0}^{N - 1}{p{1^{\prime}\lbrack {x,{- 1}} \rbrack}2}}} )\operatorname{>>}k}}} & \lbrack {{Equation}18} \rbrack\end{matrix}$${LC} = {( {{\sum\limits_{y = 0}^{N - 1}{p{1^{\prime}\lbrack {{- 1},y} \rbrack} \times p{2\lbrack {{- 1},y} \rbrack}}} + {\sum\limits_{x = 0}^{N - 1}{p{1^{\prime}\lbrack {x,{- 1}} \rbrack} \times p{2\lbrack {x,{- 1}} \rbrack}}}} )\operatorname{>>}k}$

In Equation 18, BitDepth represents a bit depth. p1′ represent a sampleof the restructured first color component, and p2 represents a sample ofthe second color component. FIG. 17 is a diagram illustrating a sampleof a first color component and a sample of a second color component.

When there is a region with no reference sample in the process ofderiving a prediction parameter, the prediction parameter can be derivedusing only existing samples.

One or more prediction parameters can be derived. For example, a firstprediction parameter may be derived from reference samples having valuessatisfying a specific requirement among reference samples used to deriveprediction parameters. In addition, a second prediction parameter may bederived from referenced samples having values that do not satisfy thespecific requirement. The specific requirement may be a condition inwhich the value of a reference sample is less than a statistic figure(for example, an average value).

According to another embodiment of the present invention, a basicprediction parameter (default parameter) may be used instead of derivinga prediction parameter from values of reference samples. The defaultparameters may be predefined in the encoder and the decoder. Forexample, the prediction parameters a and b may be respectively 1 and 0.

Alternatively, when deriving prediction parameters from referencesamples, the derived prediction parameters may be encoded and decoded.

When performing an inter color component prediction among colorcomponents Y, Cb, and Cr, prediction parameters used to predict colorcomponents Cb and Cr can be derived from a color component Y. Predictionparameters used to predict a color component Cr can be derived from acolor component Cb. Alternatively, as prediction parameters forpredicting a color component Cr, the prediction parameters that havebeen derived from a color component Y to predict a color component Cbcan be used as they are, instead of deriving new prediction parametersfor a prediction of the color component Cr.

Hereinbelow, the inter color component prediction execution step will bedescribed.

As described above, after prediction parameters are derived, an intercolor component intra prediction can be performed using at least any oneof the derived prediction parameters.

For example, a prediction of a second color component target block canbe performed by applying the derived prediction parameter to areconstructed signal of the restructured first color component,according to Equation 19.

p2[x,y]=a×p1′[x,y]+b  [Equation 19]

In Equation 19, p2[x, y] represents a prediction block of the secondcolor component target block. p1′[x, y] represents the first colorcomponent block or the restructured first color component block.

Alternatively, the prediction of the second color component target blockcan be performed by applying the derived prediction parameter to aresidual signal of the restructured first color component, according toEquation 20.

p2[x,y]=p2_pred[x,y]+a×p1′_residual[x,y]  [Equation 20]

In Equation 20, p1′_residual represents a residual signal of the firstcolor component and p2_pred represents a prediction signal obtained byperforming an intra prediction with respect to the second colorcomponent target block.

When the number of the derived prediction parameters is one or more, oneor more prediction parameters may be applied to the reconstructed sampleof the first color component. For example, when the reconstructed sampleof the first color component satisfies a specific requirement, the intercolor component intra prediction may be performed by applying the firstprediction parameter derived from the reference samples that satisfy thespecific requirement. Meanwhile, when the reconstructed sample of thefirst color component does not satisfy the specific requirement, theinter color component intra prediction may be performed by applying thesecond prediction parameter derived from the reference samples that donot satisfy the specific requirement. The specific requirement means acondition that the value of a reference sample is less than a statisticfigure (for example, an average value) of the reference samples of thefirst color component.

The inter color component prediction method may be used in an interprediction mode. For example, when performing the inter prediction onthe current block, inter prediction is performed for a first colorcomponent, and inter color component prediction or prediction combininginter prediction and inter color component prediction may be performedfor a second color component. For example, the first color component maybe a luma component, and the second color component may be a chromacomponent. In addition, the inter color component prediction may beperformed adaptively according to the coding parameters of the firstcolor component. For example, it is possible to determine whether toperform inter color component prediction according to CBF information ofthe first color component. The CBF information may be informationindicating whether a residual signal exists or not. That is, when theCBF of the first color component is 1, inter color component predictionmay be performed on the second color component. When the CBF of thefirst color component is 0, inter color component prediction may not beperformed on the second color component, and the inter prediction may beperformed on the second color component. Alternatively, a flagindicating whether or not to perform the inter color componentprediction may be signaled.

Intra prediction may be performed by combining one or more intraprediction modes. For example, an intra prediction block of the currentblock may be constructed by calculating a weighted average of blockspredicted using a predetermined non-directional intra prediction modeand blocks predicted using a predetermined directional intra predictionmode. For example, the intra prediction may be performed by calculatinga weighted sum of a value predicted using the inter color componentprediction mode and a value predicted using a predetermined intraprediction mode. In this case, the weights may vary depending on atleast one of the intra prediction mode of the current block, the size ofthe current block, the shape of the current block, and the position of apredicted sample. For example, when combining one or more intraprediction modes, a prediction block may be constructed by calculating aweighted average of a value predicted using an intra prediction mode ofthe current block and a value predicted using a predetermined modeexisting in the MPM list. When combining one or more intra predictionmodes, a representative intra prediction mode may be determined. Forexample, a process (for example, transformation and scanning) adaptivelyperformed based on the intra prediction mode of the current block may beperformed based on the representative intra prediction mode. Therepresentative intra prediction mode may be an intra prediction mode towhich a large weight is assigned. Alternatively, the one or more intraprediction modes used for the combining may be a directional mode or anon-directional mode. In the combining of one or more prediction modes,a value predicted using one intra prediction mode and a value predictedusing one inter prediction mode may be combined.

Intra prediction may be performed using one or more reference samplesets. For example, intra prediction for the current block may beperformed using a weighted average of a block obtained by performingintra prediction on unfiltered reconstructed reference samples and ablock obtained by performing intra prediction on filtered referencesamples.

In the process of performing the intra prediction, a filtering processusing neighboring reconstructed samples may be performed. Whether toperform the filtering process may be determined based on at least one ofthe intra prediction mode of the current block, the size of the currentblock, and the shape of the current block. The filtering may be includedas one step in the intra prediction process. When performing thefiltering, at least one of a filter tap, a filter coefficient, a filtershape, the number of reference sample lines to which filtering is to beapplied, and the number of samples to which filtering is to be appliedmay vary depending on at least one of the intra prediction mode, thesize, and the shape of the current block.

In the process in which the current block is divided into sub-blocks,the intra prediction mode for each of the sub-blocks is derived based onan intra prediction mode of a neighboring block, and intra predictionfor each of the sub-blocks is performed based on the derived intraprediction mode, filtering may be applied to each of the sub-blockswithin the current block. For example, a low-pass filter may be appliedto the entire area of the current block. Alternatively, a filter may beapplied to the sample located at the boundary of each sub-block.

When the current block is divided into sub-blocks and intra predictionis performed on a sub-block basis, each sub-block may be at least one ofa coding/decoding block, a prediction block, and a transform block. Forexample, when the size of the current block is 64×64 and the size of thesub-block is 16×16, the intra prediction mode may be derived for eachsub-block so that intra prediction can be performed for each sub-block.In this case, when one or more sub-blocks are further divided intosmaller 8×8 or 4×4 blocks, each 8×8 or 4×4 block may be a transformblock. In this case, the intra prediction mode of the 16×16 block may beused for prediction of each of the 8×8 or 4×4 blocks.

After performing the intra prediction to produce a prediction block, thefiltering may be applied to the prediction block. For example, at leastone of a low-pass filter and a bilateral filter may be applied. Whetherto apply the filtering may be determined based on at least one of theintra prediction mode of the current block, a position of a predictionsample, the size of the current block, and the shape of the currentblock. Alternatively, the filtering may be applied only when a specificcondition is met for each prediction sample (that is, each predictedsample). Multiple reference sample lines may be used when applying thefiltering to the prediction block. For example, when filtering theprediction samples located at the boundary between the current block andthe reference sample, the filtering may be performed using two or moreadjacent reference sample lines.

Filtering may be applied to a prediction block obtained by performingintra prediction on the current block and to a block that isreconstructed using a residual block. For example, at least one of alow-pass filter and a bilateral filter may be applied. Whether to applythe filtering may be determined based on at least one of the intraprediction mode and the size and shape of the current block.Alternatively, the filtering may be applied only when a specificcondition is met for each prediction sample (that is, each predictedsample).

The above embodiments may be performed in the same method in an encoderand a decoder.

A sequence of applying to above embodiment may be different between anencoder and a decoder, or the sequence applying to above embodiment maybe the same in the encoder and the decoder.

The above embodiment may be performed on each luma signal and chromasignal, or the above embodiment may be identically performed on luma andchroma signals.

A block form to which the above embodiments of the present invention areapplied may have a square form or a non-square form.

The above embodiment of the present invention may be applied dependingon a size of at least one of a coding block, a prediction block, atransform block, a block, a current block, a coding unit, a predictionunit, a transform unit, a unit, and a current unit. Herein, the size maybe defined as a minimum size or maximum size or both so that the aboveembodiments are applied, or may be defined as a fixed size to which theabove embodiment is applied. In addition, in the above embodiments, afirst embodiment may be applied to a first size, and a second embodimentmay be applied to a second size. In other words, the above embodimentsmay be applied in combination depending on a size. In addition, theabove embodiments may be applied when a size is equal to or greater thata minimum size and equal to or smaller than a maximum size. In otherwords, the above embodiments may be applied when a block size isincluded within a certain range.

For example, the above embodiments may be applied when a size of currentblock is 8×8 or greater. For example, the above embodiments may beapplied when a size of current block is 4×4 only. For example, the aboveembodiments may be applied when a size of current block is 16×16 orsmaller. For example, the above embodiments may be applied when a sizeof current block is equal to or greater than 16×16 and equal to orsmaller than 64×64.

The above embodiments of the present invention may be applied dependingon a temporal layer. In order to identify a temporal layer to which theabove embodiments may be applied an additional identifier may besignaled, and the above embodiments may be applied to a specifiedtemporal layer identified by the corresponding identifier. Herein, theidentifier may be defined as the lowest layer or the highest layer orboth to which the above embodiment may be applied, or may be defined toindicate a specific layer to which the embodiment is applied. Inaddition, a fixed temporal layer to which the embodiment is applied maybe defined.

For example, the above embodiments may be applied when a temporal layerof a current image is the lowest layer. For example, the aboveembodiments may be applied when a temporal layer identifier of a currentimage is 1. For example, the above embodiments may be applied when atemporal layer of a current image is the highest layer.

A slice type to which the above embodiments of the present invention areapplied may be defined, and the above embodiments may be applieddepending on the corresponding slice type.

In the above-described embodiments, the methods are described based onthe flowcharts with a series of steps or units, but the presentinvention is not limited to the order of the steps, and rather, somesteps may be performed simultaneously or in different order with othersteps. In addition, it should be appreciated by one of ordinary skill inthe art that the steps in the flowcharts do not exclude each other andthat other steps may be added to the flowcharts or some of the steps maybe deleted from the flowcharts without influencing the scope of thepresent invention.

The embodiments include various aspects of examples. All possiblecombinations for various aspects may not be described, but those skilledin the art will be able to recognize different combinations.Accordingly, the present invention may include all replacements,modifications, and changes within the scope of the claims.

The embodiments of the present invention may be implemented in a form ofprogram instructions, which are executable by various computercomponents, and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include stand-alone or acombination of program instructions, data files, data structures, etc.The program instructions recorded in the computer-readable recordingmedium may be specially designed and constructed for the presentinvention, or well-known to a person of ordinary skilled in computersoftware technology field. Examples of the computer-readable recordingmedium include magnetic recording media such as hard disks, floppydisks, and magnetic tapes; optical data storage media such as CD-ROMs orDVD-ROMs; magneto-optimum media such as floptical disks; and hardwaredevices, such as read-only memory (ROM), random-access memory (RAM),flash memory, etc., which are particularly structured to store andimplement the program instruction. Examples of the program instructionsinclude not only a mechanical language code formatted by a compiler butalso a high level language code that may be implemented by a computerusing an interpreter. The hardware devices may be configured to beoperated by one or more software modules or vice versa to conduct theprocesses according to the present invention.

Although the present invention has been described in terms of specificitems such as detailed elements as well as the limited embodiments andthe drawings, they are only provided to help more general understandingof the invention, and the present invention is not limited to the aboveembodiments. It will be appreciated by those skilled in the art to whichthe present invention pertains that various modifications and changesmay be made from the above description.

Therefore, the spirit of the present invention shall not be limited tothe above-described embodiments, and the entire scope of the appendedclaims and their equivalents will fall within the scope and spirit ofthe invention.

INDUSTRIAL APPLICABILITY

The present invention may be used in encoding/decoding an image.

1. An image decoding method performed by an image decoding apparatus,the image decoding method comprising: deriving prediction samples forthe current block based on an intra prediction mode for a current block;performing filtering on the prediction samples; and generating areconstructed picture based on residual samples for the current blockand the prediction samples, wherein based on the current block being aluma block, an intra prediction mode for the luma block is derived usingan intra prediction mode for a neighboring block of the luma block,wherein based on the intra prediction mode for the neighboring block notbeing available, the intra prediction mode for the neighboring block isset to a planar mode; and wherein the filtering is performed based onthe intra prediction mode of the current block, a size of the currentblock, and a position of a prediction sample.
 2. The image decodingmethod of claim 1, wherein the filtering is performed based on the intraprediction mode for the current block being a planar mode.
 3. The imagedecoding method of claim 1, wherein based on the current block being achroma block, an intra prediction mode for the chroma block is derivedusing an intra prediction mode of a corresponding luma positioncorresponding to a specific position of the chroma block.
 4. The imagedecoding method of claim 3, wherein based on a luma positioncorresponding to a top-left position of the chroma block being (x, y),the corresponding luma position is derived as (x+width of acorresponding luma block/2, y+height of the corresponding luma block/2).5. The image decoding method of claim 3, wherein the number of intraprediction modes for the luma block and the number of intra predictionmodes for the chroma block are different.
 6. An image encoding methodperformed by an image encoding apparatus, the image encoding methodcomprising: deriving prediction samples for the current block based onan intra prediction mode for a current block; performing filtering onthe prediction samples; deriving residual samples for the current blockbased on the prediction samples; and encoding image informationincluding residual information related to the residual samples, whereinbased on the current block being a luma block, an intra prediction modefor the luma block is derived using an intra prediction mode for aneighboring block of the luma block, wherein based on the intraprediction mode for the neighboring block not being available, the intraprediction mode for the neighboring block is set to a planar mode; andwherein the filtering is performed based on the intra prediction mode ofthe current block, a size of the current block, and a position of aprediction sample.
 7. The image encoding method of claim 6, wherein thefiltering is performed based on the intra prediction mode for thecurrent block being a planar mode.
 8. The image encoding method of claim6, wherein based on the current block being a chroma block, an intraprediction mode for the chroma block is derived using an intraprediction mode of a corresponding luma position corresponding to aspecific position of the chroma block.
 9. The image encoding method ofclaim 8, wherein based on a luma position corresponding to a top-leftposition of the chroma block being (x, y), the corresponding lumaposition is derived as (x+width of a corresponding luma block/2,y+height of the corresponding luma block/2).
 10. The image encodingmethod of claim 8, wherein the number of intra prediction modes for theluma block and the number of intra prediction modes for the chroma blockare different.
 11. A transmitting method for image data, the methodcomprising: obtaining a bitstream of encoded image information, whereinthe encoded image information is generated based on deriving predictionsamples for the current block based on an intra prediction mode for acurrent block, performing filtering on the prediction sample, derivingresidual samples for the current block based on the prediction samples,and encoding image information including residual information related tothe residual samples, and transmitting the image data comprising thebitstream, wherein based on the current block being a luma block, anintra prediction mode for the luma block is derived using an intraprediction mode for a neighboring block of the luma block, wherein basedon the intra prediction mode for the neighboring block not beingavailable, the intra prediction mode for the neighboring block is set toa planar mode; and wherein the filtering is performed based on the intraprediction mode of the current block, a size of the current block, and aposition of a prediction sample.